fix: Accept Path-like objects in credential factory functions

[hiranya911]

Credential factory functions like credentials.cert() currently only accept str typed file paths. But Python 3.4 introduced the pathlib module and with that a wide range of path-like types. This PR makes it possible to pass these path-like objects directly to our credential factory functions.

import pathlib

path = pathlib.Path('home', 'user', 'firebase.json')
cred = credentials.Cert(path)

Basically we are going to treat anything that is accepted by pathlib.Path as a valid file path. This constructor has good built-in argument validation, and throws TypeError for things that not path-like.

Resolves #501

[hiranya911]

As suspected encountering issues with Python 3.5. Instead of implementing an ugly hack, I think a better course of action is to drop Python 3.5 support (which EoL'ed a couple of months ago), and delay this fix until then.

[lahirumaramba]

Great! Thanks, Hiranya!