PBKDF2 type hint matches code

[yorickdowne]

This PR narrowly scratches an itch in my code base, where I need a type ignore for PBKDF2. We can discuss widening it to PBKDF1 and scrypt

PBKDF2 is currently type-hinted with password: str, salt: bytes, but the code does

     password = tobytes(password)
     salt = tobytes(salt)

and the doc string agrees

 password (string or byte string):
 salt (string or byte string):

Potential extension

PBKDF1 also does password = tobytes(password) , so password at last could by bytes or str. The salt is just appended, it must be bytes. This would however also require changing the doc from currently password (string): to password (string or byte string):

scrypt delegates to PBKDF2 with stage_1 = PBKDF2(password, salt, p * 128 * r, 1, prf=prf_hmac_sha256) so bytes absolutely work, but docs and type hints say str. Could be changed in line with PBKDF2

Note tobytes agrees that it could be bytes or str: def tobytes(s: Union[bytes, str]) -> bytes: