load_pem_x509_cert_str_safe: Handle str or bytes input

[zmousm]

• Introduce utils.load_pem_x509_cert_str_safe to handle str or bytes as input to load_pem_x509_certificate
• Use it to wrap calls to the latter where input can be str or bytes

All Submissions:

• [X] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [X] Have you added an explanation of what problem you are trying to solve with this PR?
• [X] 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]

I opted to make the least changes and only wrap calls to load_pem_x509_certificate where the input may not be a bytestring