fix pycrypto to fit pycryptodome

[sot528]

When I use pyjwt with PyCryptodome, I got an error.

AttributeError: module 'Crypto.PublicKey.RSA' has no attribute '_RSAobj'

Traceback (most recent call last):
  File "tmp.py", line 19, in <module>
    audience='dj00aiZpPW9MRGxxxxxxxxxxxxxxxxxxxxxxx-',
  File "/foo/bar/venv/lib/python3.6/site-packages/jwt/api_jwt.py", line 92, in decode
    jwt, key=key, algorithms=algorithms, options=options, **kwargs
  File "/foo/bar/venv/lib/python3.6/site-packages/jwt/api_jws.py", line 156, in decode
    key, algorithms)
  File "/foo/bar/venv/lib/python3.6/site-packages/jwt/api_jws.py", line 220, in _verify_signature
    key = alg_obj.prepare_key(key)
  File "/foo/bar/venv/lib/python3.6/site-packages/jwt/contrib/algorithms/pycrypto.py", line 29, in prepare_key
    if isinstance(key, RSA._RSAobj):

It because of un compatibility between PyCrypto with PyCryptodome.
So I fixed it.

[coveralls]

Pull Request Test Coverage Report for Build 66

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 76.741%

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Totals
Change from base Build 62:	0.0%
Covered Lines:	551
Relevant Lines:	718

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💛 - Coveralls

[jpadilla]

@sot528 any chance you could add a test or two to document the original problem and solution?

Also, this might also be a good idea to make sure we have official support for pycryptodome just like we do for cryptography.

[jpadilla]

Addressing this in #470