Log Message:
2.1.2
Fix README rendering.

2.1.1
Restored use of ``portend.client_host`` during
    ``assert_free`` check on Windows, fixing check
    when the bind address is *ADDR_ANY.

2.1
Use tempora.timing.Timer from tempora 1.8, replacing
boilerplate code in occupied and free functions.

2.0
Removed ``portend._getaddrinfo`` and its usage in
    ``Checker.assert_free``.

Dropped support for Python 2.6.