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

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[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