Adapt to cryptography 2.7's deprecations

[njsmith]

https://travis-ci.org/python-trio/trio/jobs/540515155:

../../../../virtualenv/python3.6.7/lib/python3.6/site-packages/trio/tests/test_ssl.py:52: in <module>
    TRIO_TEST_1_CERT = TRIO_TEST_CA.issue_server_cert("trio-test-1.example.org")
../../../../virtualenv/python3.6.7/lib/python3.6/site-packages/trustme/__init__.py:359: in issue_cert
    ski),
../../../../virtualenv/python3.6.7/lib/python3.6/site-packages/cryptography/x509/extensions.py:202: in from_issuer_subject_key_identifier
    stacklevel=2,
E   cryptography.utils.CryptographyDeprecationWarning: Extension objects are deprecated as arguments to from_issuer_subject_key_identifier and support will be removed soon. Please migrate to passing a SubjectKeyIdentifier directly.

[anthraxx]

@njsmith Any way we can get this fixed?
Lowering cryptography dependency is not a workable solution for our distro packages :/

[njsmith]

@anthraxx Yeah, it's definitely not a good solution. It's only a warning, so your distro could just ignore it for now. Alternatively, if you'd like to submit a PR, that would be great!

[pquentin]

@njsmith fixed that bug in #50 and then released 0.5.2. We just forgot to close that issue! Doing it now.

[njsmith]

Oh whoops, I totally forgot about that :-)

@anthraxx are there some other warnings you're hitting...?

[anthraxx]

Oh, missed the 0.5.2 upgrade, all fine now, thanks!
@njsmith was not easily ignoreable, because the warnings triggered test failures on other projects :)