Add controls for verify_sub option in PyJWT

[jlucier]

This PR addresses the discussion in this issue: #561

For people who have historically been using values for the sub claim other than strings, they may need or want the ability to control PyJWT's new behavior (as of 2.10.0) which is to reject those tokens as invalid.

This work adds a new option JWT_VERIFY_SUB which can be used to control the verify_sub option when calling jwt.decode(...) from the PyJWT library.

The default value for JWT_VERIFY_SUB is True, which results in the sub claim being verified. This default results in no change to how the library behaved prior to this addition. By choosing to set JWT_VERIFY_SUB to False, the user can turn off the verify_sub option in the jwt.decode call.

I welcome feedback on the naming or anything else regarding the implementation. Also, please let me know if I missed any places where this needs to be tested or documented. Thanks!

[vimalloc]

Looks great, thanks for putting this together!