* Fix weak linking of getentropy when compiling on older macOS

We use weak linking in macOS to determine if the getentropy symbol is
available. However, to do that we need to have a declaration that states
the function is __attribute((weak_import)) at compile time. On macOS
10.12 this is provided in sys/random.h, but on older macOS the
declaration doesn't exist at all, so we need to forward declare it
ourselves.

* update a comment and a style nit

* Blacklist pytest 3.2.0 because it breaks us (#3835)

See upstream: https://github.com/pytest-dev/pytest/issues/2644

* fix for latest flake8 (#3829)

* changelog and version bump for 2.0.2

* diff language

the getentropy fallback is only possible on macOS, wrap it in a define
to remove it entirely on the BSDs.

* 2.0 version bump and changelog

* dates are just an illusion

* Remove DH generator size constraint

* Check that g > 1

* add AESGCM AEAD support

* remove stray newline

* move AESGCM docs above CCM