install: remove need to name [esp]

Simplify install to just `pip install django-anymail`.
(Rather than `... django-anymail[mailgun]`

All of the ESPs so far require requests, so just move
that into the base requirements. (Chances are your
Django app already needs requests for some other
reason, anyway.)

Truly unique ESP dependencies (e.g., boto for
AWS-SES) could still use the setup extra features
mechanism.

anymail/backends/base_requests.py

@@ -1,18 +1,13 @@
 import json
 
+import requests
 # noinspection PyUnresolvedReferences
 from six.moves.urllib.parse import urljoin
 
 from .base import AnymailBaseBackend, BasePayload
-from ..exceptions import AnymailImproperlyInstalled, AnymailRequestsAPIError, AnymailSerializationError
+from ..exceptions import AnymailRequestsAPIError, AnymailSerializationError
 from .._version import __version__
 
-try:
-    # noinspection PyUnresolvedReferences
-    import requests
-except ImportError:
-    raise AnymailImproperlyInstalled('requests')
-
 
 class AnymailRequestsBackend(AnymailBaseBackend):
     """