Fix mypy errors

[jpadilla]

mypy create: /home/travis/build/jpadilla/pyjwt/.tox/mypy
mypy inst: /home/travis/build/jpadilla/pyjwt/.tox/.tmp/package/1/PyJWT-1.7.1.zip
mypy installed: mypy==0.740,mypy-extensions==0.4.3,PyJWT==1.7.1,typed-ast==1.4.0,typing-extensions==3.7.4
mypy run-test-pre: PYTHONHASHSEED='2425519668'
mypy run-test: commands[0] | mypy --ignore-missing-imports jwt
jwt/help.py:12: error: Incompatible types in assignment (expression has type "None", variable has type Module)
Found 1 error in 1 file (checked 13 source files)
ERROR: InvocationError for command /home/travis/build/jpadilla/pyjwt/.tox/mypy/bin/mypy --ignore-missing-imports jwt (exited with code 1)

[coveralls]

Pull Request Test Coverage Report for Build 86

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• 63 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-8.4%) to 68.011%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
jwt/help.py	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
jwt/compat.py	11	51.28%
jwt/contrib/algorithms/pycrypto.py	25	3.85%
jwt/contrib/algorithms/py_ecdsa.py	27	6.9%

Totals
Change from base Build 80:	-8.4%
Covered Lines:	489
Relevant Lines:	719

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💛 - Coveralls