Bump pyOpenSSL dependency on Python 2.7

[kislyuk]

This avoids incompatibilities with older versions of OpenSSL and errors like this:

AttributeError: 'module' object has no attribute 'SSL_ST_INIT'

[ttung]

This wasn't an explicit requirement before, right? Why is it now?

[codecov-io]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   87.93%   87.93%           
=======================================
  Files          29       29           
  Lines         970      970           
=======================================
  Hits          853      853           
  Misses        117      117

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[kislyuk]

PyOpenSSL is an optional transitive requirement of requests. It always tries to import and use pyOpenSSL instead of the native Python OpenSSL bindings, but silently proceeds if that import fails.

On Python < 2.7.9 or so, SNI is not supported by the native Python OpenSSL bindings. There are other issues with it as well. That's why it tries to do that.

PyOpenSSL < 16.2.0 induces this crash when loading OpenSSL 1.1.x. So basically if pyOpenSSL is present, we need it to be up to date.

https://github.com/requests/requests/blob/master/requests/__init__.py#L93-L102