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.

setup.py

@@ -31,11 +31,15 @@ setup(
     license="BSD License",
     packages=["anymail"],
     zip_safe=False,
-    install_requires=["django>=1.8", "six"],
+    install_requires=["django>=1.8", "requests>=2.4.3", "six"],
     extras_require={
-        "mailgun": ["requests>=2.4.3"],
-        "mandrill": ["requests>=1.0.0"],
-        "sendgrid": ["requests>=2.4.3"],
+        # This can be used if particular backends have unique dependencies
+        # (e.g., AWS-SES would want boto).
+        # For simplicity, requests is included in the base requirements.
+        "mailgun": [],
+        "mandrill": [],
+        "postmark": [],
+        "sendgrid": [],
     },
     include_package_data=True,
     test_suite="runtests.runtests",