PyJWKClient raises error when key omits "use" (Public Key Use) parameter

[mjr9804]

PyJWKClient.get_signing_keys is hard-coded to look for keys that have a use (Public Key Use) value of sig. This causes errors when working with JWKs that omit this parameter. I believe this is an optional parameter according to RFC 7517, Section 4.2.

Adjusting the check at https://github.com/jpadilla/pyjwt/blob/master/jwt/jwks_client.py#L32 fixes the problem, but I'm not sure what the best solution is. It seems you could either assume that a key that omits use is a signing key, ignore any keys with "use": "enc", or maybe expose some configuration knob.

Expected Result

PyJWKClient should be able to parse signing keys from JWKs that omit the optional use parameter.

Actual Result

A PyJWKClientError is raised.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../Python/3.7/lib/python/site-packages/jwt/jwks_client.py", line 31, in get_signing_keys
    raise PyJWKClientError("The JWKS endpoint did not contain any signing keys")
jwt.exceptions.PyJWKClientError: The JWKS endpoint did not contain any signing keys

Reproduction Steps

import jwt

# Sample JWK that omits 'use'
SAMPLE_URL = 'https://gist.githubusercontent.com/mjr9804/83e8388cc5d2a03e54938da189668770/raw/5514eec3c12714eb2a57bf180810b23ac841ad9b/jwk.json'
jwks_client = jwt.PyJWKClient(SAMPLE_URL)
jwks_client.get_signing_keys()

System Information

$ python -m jwt.help

{
    "cryptography": {
      "version": "3.2"
    },
    "implementation": {
      "name": "CPython",
      "version": "3.7.3"
    },
    "platform": {
      "release": "19.6.0",
      "system": "Darwin"
    },
    "pyjwt": {
      "version": "2.0.1"
    }
}

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