Remove cryptography, pyopenssl and idna from requirements

[czoido]

Changelog: Fix: Remove cryptography, pyopenssl and idna from OSX requirements in Python.
Docs: omit

Closes: #8061
Close: #7286

As summarised in this comment, looks like it would be safe to remove these 3 Python dependencies:

• idna was introduced because installing via requests was installing an incorrect version but looks like the issue is not happening any more so it should be safe to remove.
• pyopenssl and cryptography were introduced just for OSX here: memsharded/conan@10d69ff because of Downloads from certain SSL domains fails in OSX #309

The only cases that might fail is for Python<2.7.9 and a very old openssl version in the host platform. In that case this could be solved by doing python -m pip install pyopenssl cryptography idna so it looks relatively safe to remove this dependencies.

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#TAGS: svn, slow
#REVISIONS: 1