Fix load pem private key with the PEM encoded data

[g1im2]

error will be happened with “initializer for ctype 'char[]' must be a bytes or list or tuple, not str”，fix it like pyca/cryptography#2730

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[coveralls]

Coverage increased (+0.08%) to 43.704% when pulling bec3e64 on g1im2:master into 0e50795 on google:master.