Fix load pem private key with the PEM encoded data

[g1im2]

fix problem like this issues "pyca/cryptography#2730", and it also happened in here.

[coveralls]

Coverage remained the same at 43.623% when pulling f1c3d24 on g1im2:master into 0e50795 on google:master.

[greateggsgreg]

This definitely needs to be fixed. An alternative method is to open the files in binary (mode "rb") so we don't need to convert to bytes after the fact. Example:

with open(rsa_key_path + '.pub', 'rb') as rsa_pub_file:
       self.public_key = rsa_pub_file.read()

with open(rsa_key_path, 'rb') as rsa_prv_file:
       self.rsa_key = serialization.load_pem_private_key(
             rsa_prv_file.read(), None, default_backend())

[JeffLIrion]

I created a new package adb-shell (see #167) and incorporated @greateggsgreg's changes into sign_cryptography.py.

However, the CryptographySigner.Sign method raises an exception, and in the test_sign test I'm actually checking that the Sign method is broken. By contrast, the corresponding tests for PycryptodomeAuthSigner.Sign and PythonRSASigner.Sign are identical (aside from the exception catching) and they pass just fine.

If you submit a pull request that fixes this issue, that would be much appreciated!