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/docs/installation.rst b/docs/installation.rst
index f96cc50..13cd75e 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -10,7 +10,7 @@
 Supported platforms
 -------------------
 
-Currently we test ``cryptography`` on Python 2.7, 3.4, 3.5, 3.6, and
+Currently we test ``cryptography`` on Python 2.7, 3.4+, and
 PyPy 5.3+ on these operating systems.
 
 * x86-64 CentOS 7.x