Creating custom token fails

Hey, First, thanks for pyrebase.

I was getting the following error/logtrace :


    return jwt.generate_jwt(payload, private_key, "RS256", exp)
  File "/Users/guyma/development/firebase-poc/venv/lib/python3.10/site-packages/python_jwt/__init__.py", line 91, in generate_jwt
    token.add_signature(priv_key, protected=header)
  File "/Users/guyma/development/firebase-poc/venv/lib/python3.10/site-packages/jwcrypto/jws.py", line 540, in add_signature
    sig = c.sign()
  File "/Users/guyma/development/firebase-poc/venv/lib/python3.10/site-packages/jwcrypto/jws.py", line 149, in sign
    raise ValueError('key is not a JWK object')
ValueError: key is not a JWK object```

**I managed to solve it locally by replacing line 104~ in pyrebase.py**
from:         `private_key = RSA.importKey(self.credentials._private_key_pkcs8_pem)`
to:             `private_key = jwk.JWK.from_pem(str.encode(self.credentials._private_key_pkcs8_pem))`

Thank you!
---
Make sure these boxes are checked before submitting your issue:

[V] Check that your version of Python is 3.4+
[V] Check that you are on the newest version of Pyrebase
[V] Check that Email/password provider is enabled in your Firebase dashboard under Auth -> Sign In Method.

Please don't be discouraged if you do not get a response to your issue quickly,
I maintain Pyrebase for fun and don't always have as much free time as I'd like.

Thank you for helping make Pyrebase better!

nhorvath / Pyrebase4

Creating custom token fails #55