Catch and re-raise requests.RequestException in
AnymailRequestsBackend.post_to_esp.
* AnymailRequestsAPIError is needed for proper
fail_silently handling.
* Retain original requests exception type, to avoid
breaking existing code that might look for specific
requests exceptions.
Closes#16
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.
(Prep for installing backends as package extras)
* Extract AnymailRequestsBackend and RequestsPayload
to base_requests.py
* Don't define/require requests exceptions when requests
not available
