[SDK][Python] fix: encryption module now can work not only with strings

[Dzeranov]

Description

Encryption module now can work not only with strings

Summary of changes

1. Both sign_and_encrypt and sign methods now can work both with str and bytes types.
2. decrypt method now returns bytes instead of str

Related issues

Part of the #2717

NOTE: Don't merge this PR.

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[dnechay]

LGTM overall, 2 questions:

• does it make sense to add a unit test on encrypt/decrypt of actual bytes? Maybe some dumb object converted to bytes
• do we expect the same 1:1 behavior from JS sdk, i.e. decrypt to return binary if encrypted message is string and encrypt to accept both? I thought for these methods we were going to use just bytes and potentially provide some convenience methods like encryptStrMessage, decryptStrMessage

[Dzeranov]

@dnechay

1. I'll add it, this PR is not ready to be merged yet, simply to show what will be the behavior.
2. decrypt method returns bytes in this implementation for both str and bytes. As for input parameters, pgpy package doesn't care what is the data you need to encrypt, Union[str, bytes] is simply for a type hint here.