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/sendgrid.py

@@ -1,17 +1,13 @@
 from django.core.exceptions import ImproperlyConfigured
 from django.core.mail import make_msgid
+from requests.structures import CaseInsensitiveDict
 
-from ..exceptions import AnymailImproperlyInstalled, AnymailRequestsAPIError
+from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
 from ..utils import get_anymail_setting, timestamp
 
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 
-try:
-    # noinspection PyUnresolvedReferences
-    from requests.structures import CaseInsensitiveDict
-except ImportError:
-    raise AnymailImproperlyInstalled('requests', backend="sendgrid")
 
 
 class SendGridBackend(AnymailRequestsBackend):