Created as a new function as it will be needed by the new PKCS#1 2.0 implementation. Specifically, for the MGF1 function used in the OAEP encoding/decoding.
This allows us not to have `math` dependencies

Good catch, thanks!

Now that we no longer support Python 2.6, we can use int.bit_length()
instead. Co-authored by @adamantike.

This makes catching exceptions slightly stronger, as it is now possible
to check for this specific exception. Furthermore, information about the
not-prime numbers is included in the exception object.

This unittest tests both execution branches of the function, reducing
randomness of code coverage.

Also:
- changed http to https in the code
- changed header underlines in the documentation to match the header length

Mostly focused on docstrings (''' → """), indentation, empty lines,
and superfluous parenthesis.