Let crypto._cert_fingerprint pass bytes to load_pem_x509_certificate

[zmousm]

crypto._cert_fingerprint must ensure first parameter to load_pem_x509_certificate is bytes, as there are code paths which pass this as a string (such as crypto.from_keyspec -> crypto._load_keyspec -> crypto.XMLSecCryptoFromXML.__init__ -> crypto._cert_fingerprint).

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[zmousm]

The issue affecting crypto._cert_fingerprint also exists in other places where load_pem_x509_certificate is called, so I think #78 may be a better solution