Move away from deprecated methods

[nelz9999]

I noticed that running the project unit test in pytest, I was getting some of the following Warnings:

tests/algorithms/test_EC.py::TestECAlgorithm::()::test_signing_parity[CryptographyECKey-ECDSAECKey]
  ~/python-jose/jose/backends/cryptography_backend.py:104: CryptographyDeprecationWarning: signer and verifier have been deprecated. Please use sign and verify instead.
    verifier = self.prepared_key.verifier(signature, ec.ECDSA(self.hash_alg()))

tests/algorithms/test_RSA.py::TestRSAAlgorithm::()::test_signing_parity[CryptographyRSAKey-CryptographyRSAKey]
  ~/python-jose/jose/backends/cryptography_backend.py:257: CryptographyDeprecationWarning: signer and verifier have been deprecated. Please use sign and verify instead.
    self.hash_alg()
  ~/python-jose/jose/backends/cryptography_backend.py:269: CryptographyDeprecationWarning: signer and verifier have been deprecated. Please use sign and verify instead.
    self.hash_alg()

This PR moves from verifier->verify and signer->sign.

I would love a decent review of this code, just to double check that I didn't borken anything.

[codecov-io]

Codecov Report

Merging #85 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   93.69%   93.65%   -0.05%     
==========================================
  Files          12       12              
  Lines         841      835       -6     
==========================================
- Hits          788      782       -6     
  Misses         53       53

Impacted Files	Coverage Δ
jose/backends/cryptography_backend.py	100% <100%> (ø)	⬆️

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[zejn]

Looks good to me. /cc @mpdavis

[mpdavis]

lgtm