Add support of python 3.8

[Djailla]

Python 3.8 is out, let's make pyjwt CI and package compatible

[coveralls]

Pull Request Test Coverage Report for Build 110

• 0 of 0 changed or added relevant lines in 0 files are covered.
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• Overall coverage remained the same at ?%

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[Djailla]

@jpadilla Hello, could you take a look at my PR, especially the error I got with python 3.8 with incompatibility with pycrypto (no longer supported library) using time.clock deprecated in Python 3.8.

Thanks !

[Djailla]

Here is the stack trace :

_ TestPycryptoAlgorithms.test_rsa_sign_should_generate_correct_signature_value _

self = <tests.contrib.test_algorithms.TestPycryptoAlgorithms object at 0x7fba84ebf2e0>

    def test_rsa_sign_should_generate_correct_signature_value(self):
        algo = RSAAlgorithm(RSAAlgorithm.SHA256)
    
        jwt_message = force_bytes("Hello World!")
    
        expected_sig = base64.b64decode(
            force_bytes(
                "yS6zk9DBkuGTtcBzLUzSpo9gGJxJFOGvUqN01iLhWHrzBQ9ZEz3+Ae38AXp"
                "10RWwscp42ySC85Z6zoN67yGkLNWnfmCZSEv+xqELGEvBJvciOKsrhiObUl"
                "2mveSc1oeO/2ujkGDkkkJ2epn0YliacVjZF5+/uDmImUfAAj8lzjnHlzYix"
                "sn5jGz1H07jYYbi9diixN8IUhXeTafwFg02IcONhum29V40Wu6O5tAKWlJX"
                "fHJnNUzAEUOXS0WahHVb57D30pcgIji9z923q90p5c7E2cU8V+E1qe8NdCA"
                "APCDzZZ9zQ/dgcMVaBrGrgimrcLbPjueOKFgSO+SSjIElKA=="
            )
        )
    
        with open(key_path("testkey_rsa"), "r") as keyfile:
            jwt_key = algo.prepare_key(keyfile.read())
    
        with open(key_path("testkey_rsa.pub"), "r") as keyfile:
            jwt_pub_key = algo.prepare_key(keyfile.read())
    
>       algo.sign(jwt_message, jwt_key)

tests/contrib/test_algorithms.py:68: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
jwt/contrib/algorithms/pycrypto.py:44: in sign
    return PKCS1_v1_5.new(key).sign(self.hash_alg.new(msg))
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Signature/PKCS1_v1_5.py:112: in sign
    m = self._key.decrypt(em)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/PublicKey/RSA.py:174: in decrypt
    return pubkey.pubkey.decrypt(self, ciphertext)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/PublicKey/pubkey.py:93: in decrypt
    plaintext=self._decrypt(ciphertext)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/PublicKey/RSA.py:235: in _decrypt
    r = getRandomRange(1, self.key.n-1, randfunc=self._randfunc)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Util/number.py:123: in getRandomRange
    value = getRandomInteger(bits, randfunc)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Util/number.py:104: in getRandomInteger
    S = randfunc(N>>3)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Random/_UserFriendlyRNG.py:202: in read
    return self._singleton.read(bytes)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Random/_UserFriendlyRNG.py:178: in read
    return _UserFriendlyRNG.read(self, bytes)
.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Random/_UserFriendlyRNG.py:129: in read
    self._ec.collect()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <Crypto.Random._UserFriendlyRNG._EntropyCollector object at 0x7fba84ebfd30>

    def collect(self):
        # Collect 64 bits of entropy from the operating system and feed it to Fortuna.
        self._osrng_es.feed(self._osrng.read(8))
    
        # Add the fractional part of time.time()
        t = time.time()
        self._time_es.feed(struct.pack("@I", int(2**30 * (t - floor(t)))))
    
        # Add the fractional part of time.clock()
>       t = time.clock()
E       AttributeError: module 'time' has no attribute 'clock'

.tox/py38-contrib_crypto/lib/python3.8/site-packages/Crypto/Random/_UserFriendlyRNG.py:77: AttributeError```

[tirkarthi]

The error due to time.clock is an upstream issue : pycrypto/pycrypto#283 . The upstream package is not maintained so it would make sense to look into pycryptodome as noted in the message for compatibility.

Thanks.