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/exceptions.py

@@ -1,14 +1,7 @@
 import json
 
 from django.core.exceptions import ImproperlyConfigured
-
-try:
-    from requests import HTTPError
-except ImportError:
-    # Backends that don't use requests aren't required to have it installed
-    # (and could never raise an AnymailRequestsAPIError)
-    class HTTPError(Exception):
-        pass
+from requests import HTTPError
 
 
 class AnymailError(Exception):