Drop Python 2 and Django 1.11 support

Minimum supported versions are now Django 2.0, Python 3.5.

This touches a lot of code, to:
* Remove obsolete portability code and workarounds
  (six, backports of email parsers, test utils, etc.)
* Use Python 3 syntax (class defs, raise ... from, etc.)
* Correct inheritance for mixin classes
* Fix outdated docs content and links
* Suppress Python 3 "unclosed SSLSocket" ResourceWarnings
  that are beyond our control (in integration tests due to boto3, 
  python-sparkpost)

anymail/inbound.py

@@ -1,15 +1,15 @@
 from base64 import b64decode
 from email.message import Message
+from email.parser import BytesParser, Parser
+from email.policy import default as default_policy
 from email.utils import unquote
 
-import six
 from django.core.files.uploadedfile import SimpleUploadedFile
 
-from ._email_compat import EmailParser, EmailBytesParser
-from .utils import angle_wrap, get_content_disposition, parse_address_list, parse_rfc2822date
+from .utils import angle_wrap, parse_address_list, parse_rfc2822date
 
 
-class AnymailInboundMessage(Message, object):  # `object` ensures new-style class in Python 2)
+class AnymailInboundMessage(Message):
     """
     A normalized, parsed inbound email message.
 
@@ -31,7 +31,7 @@ class AnymailInboundMessage(Message, object):  # `object` ensures new-style clas
 
     def __init__(self, *args, **kwargs):
         # Note: this must accept zero arguments, for use with message_from_string (email.parser)
-        super(AnymailInboundMessage, self).__init__(*args, **kwargs)
+        super().__init__(*args, **kwargs)
 
         # Additional attrs provided by some ESPs:
         self.envelope_sender = None
@@ -125,14 +125,7 @@ class AnymailInboundMessage(Message, object):  # `object` ensures new-style clas
                 return part.get_content_text()
         return None
 
-    # Backport from Python 3.5 email.message.Message
-    def get_content_disposition(self):
-        try:
-            return super(AnymailInboundMessage, self).get_content_disposition()
-        except AttributeError:
-            return get_content_disposition(self)
-
-    # Backport from Python 3.4.2 email.message.MIMEPart
+    # Hoisted from email.message.MIMEPart
     def is_attachment(self):
         return self.get_content_disposition() == 'attachment'
 
@@ -148,10 +141,7 @@ class AnymailInboundMessage(Message, object):  # `object` ensures new-style clas
             # (Note that self.is_multipart() misleadingly returns True in this case.)
             payload = self.get_payload()
             assert len(payload) == 1  # should be exactly one message
-            try:
-                return payload[0].as_bytes()  # Python 3
-            except AttributeError:
-                return payload[0].as_string().encode('utf-8')
+            return payload[0].as_bytes()
         elif maintype == 'multipart':
             # The attachment itself is multipart; the payload is a list of parts,
             # and it's not clear which one is the "content".
@@ -199,24 +189,24 @@ class AnymailInboundMessage(Message, object):  # `object` ensures new-style clas
     @classmethod
     def parse_raw_mime(cls, s):
         """Returns a new AnymailInboundMessage parsed from str s"""
-        if isinstance(s, six.text_type):
+        if isinstance(s, str):
             # Avoid Python 3.x issue https://bugs.python.org/issue18271
             # (See test_inbound: test_parse_raw_mime_8bit_utf8)
             return cls.parse_raw_mime_bytes(s.encode('utf-8'))
-        return EmailParser(cls).parsestr(s)
+        return Parser(cls, policy=default_policy).parsestr(s)
 
     @classmethod
     def parse_raw_mime_bytes(cls, b):
         """Returns a new AnymailInboundMessage parsed from bytes b"""
-        return EmailBytesParser(cls).parsebytes(b)
+        return BytesParser(cls, policy=default_policy).parsebytes(b)
 
     @classmethod
     def parse_raw_mime_file(cls, fp):
         """Returns a new AnymailInboundMessage parsed from file-like object fp"""
-        if isinstance(fp.read(0), six.binary_type):
-            return EmailBytesParser(cls).parse(fp)
+        if isinstance(fp.read(0), bytes):
+            return BytesParser(cls, policy=default_policy).parse(fp)
         else:
-            return EmailParser(cls).parse(fp)
+            return Parser(cls, policy=default_policy).parse(fp)
 
     @classmethod
     def construct(cls, raw_headers=None, from_email=None, to=None, cc=None, subject=None, headers=None,
@@ -242,7 +232,7 @@ class AnymailInboundMessage(Message, object):  # `object` ensures new-style clas
         :return: {AnymailInboundMessage}
         """
         if raw_headers is not None:
-            msg = EmailParser(cls).parsestr(raw_headers, headersonly=True)
+            msg = Parser(cls, policy=default_policy).parsestr(raw_headers, headersonly=True)
             msg.set_payload(None)  # headersonly forces an empty string payload, which breaks things later
         else:
             msg = cls()
@@ -336,7 +326,7 @@ class AnymailInboundMessage(Message, object):  # `object` ensures new-style clas
         if part.get_content_maintype() == 'message':
             # email.Message parses message/rfc822 parts as a "multipart" (list) payload
             # whose single item is the recursively-parsed message attachment
-            if isinstance(content, six.binary_type):
+            if isinstance(content, bytes):
                 content = content.decode()
             payload = [cls.parse_raw_mime(content)]
             charset = None