Add py37 (#4298)

* test against python 3.7 for windows

* update docs to say we test on 3.7

* more succinct

* maybe make this actually work.

* link properly

* moar changes
diff --git a/setup.py b/setup.py
index 593925e..2fbbd04 100644
--- a/setup.py
+++ b/setup.py
@@ -273,6 +273,7 @@
         "Programming Language :: Python :: 3.4",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: Implementation :: CPython",
         "Programming Language :: Python :: Implementation :: PyPy",
         "Topic :: Security :: Cryptography",