Set sign algo acording to key type

[skorobkov]

When client uses exisiting ACME account with private key NOT RSA type, on requests to ACME server he will get error:

  File "<console>", line 1, in <module>
  File "/opt/venv/lib/python3.8/site-packages/acme/client.py", line 138, in new_order
    response = self._post(self.directory['newOrder'], order)
  File "/opt/venv/lib/python3.8/site-packages/acme/client.py", line 366, in _post
    return self.net.post(*args, **kwargs)
  File "/opt/venv/lib/python3.8/site-packages/acme/client.py", line 739, in post
    return self._post_once(*args, **kwargs)
  File "/opt/venv/lib/python3.8/site-packages/acme/client.py", line 749, in _post_once
    data = self._wrap_in_jws(obj, self._get_nonce(url, new_nonce_url), url)
  File "/opt/venv/lib/python3.8/site-packages/acme/client.py", line 560, in _wrap_in_jws
    return jws.JWS.sign(jobj, **cast(Mapping[str, Any], kwargs)).json_dumps(indent=2)
  File "/opt/venv/lib/python3.8/site-packages/acme/jws.py", line 56, in sign
    return super().sign(payload, key=key, alg=alg,
  File "/opt/venv/lib/python3.8/site-packages/josepy/jws.py", line 291, in sign
    cls.signature_cls.sign(payload=payload, **kwargs),))
  File "/opt/venv/lib/python3.8/site-packages/josepy/jws.py", line 231, in sign
    assert isinstance(key, alg.kty)
AssertionError

This error occurs because acme.client.ClientNetwork initialised with default RS256 sign algorithm.
This PR is fixing this issue (at least for EC keys).

[hosseinsh]

Thanks for the PR @skorobkov.
Would appreciate it if you get a chance to resolve the Linting-related errors.

--> Linting Python files
PYFLAKES_NODOCTEST=1 flake8 lemur
lemur/common/utils.py:248:1: E302 expected 2 blank lines, found 1
make: *** [Makefile:96: lint-python] Error 1
Error: Process completed with exit code 2.

[coveralls]

coverage: 75.407% (+0.008%) from 75.399% when pulling a68c4d0 on skorobkov:acme_clinet_sign_alg into b6d3531 on Netflix:master.