Drop support for pyOpenSSL < 0.10

[exarkun]

In #4854, we added support for using the memory bio features of pyOpenSSL >= 0.10 to implement IReactorSSL in all reactors.

In #4974, we will deprecate the support based on pyOpenSSL < 0.10.

After that deprecation has been in place for some time, we should remove support for working with pyOpenSSL < 0.10.

If #4974 is resolved for 11.1, then I think 12.1 would be the right time to do this. When we do, we can get rid of all of twisted/internet/_oldtls.py and the conditionals elsewhere which import that module when pyOpenSSL < 0.10 is in use.

[exarkun]

(In [31537]) Merge protocol-ssl-4854-6

Author: exarkun Reviewer: itamar, glyph, thijs Fixes: #4854 Refs: #4974 Refs: #5014 Refs: #4455

Add an implementation of IReactorSSL and ITLSTransport which uses the memory BIO APIs present in pyOpenSSL 0.10 and newer. This implementation will be preferred by all reactors if the pyOpenSSL dependency is satisfied, otherwise the old implementation will still be used.

This appears to have slightly better performance than the old implementation and should avoid bugs like #4455.

[itamar]

Removing 12.1 milestone, since #4974 is still outstanding.