Accelerate YAML parsing when LibYAML is available

[hiromu]

When a Swagger definition is provided in YAML, the current implementation uses a Python loader for parsing.
However, it can be accelerated by using a native loader when LibYAML is available.

[coveralls]

Coverage decreased (-0.1%) to 99.074% when pulling bfa66ab on hiromu:enable-libyaml-loader into 3f04ce5 on Yelp:master.

[sjaensch]

Generally looks good to me, could you fix the pre-commit and mypy issue? I'll be happy to merge it in once the build passes.

[hiromu]

Thank you for your reply. I updated this pull request to pass all the tests.

For the mypy issue, I simply skipped the line in the same manner as people discussing in the mypy repository: python/mypy#1153

[sjaensch]

I don't think we should modify the standard py37 test environment for that case. Either we create a separate one or we just don't test this case.

[sjaensch]

Feel free to just remove your changes to this file, and I will merge your branch.

[hiromu]

I had trouble keeping the coverage check and figured out that we should modify test environments to cover the case that native libyaml is not available.
Thus, as you suggested, I will separate the case as a new environment.

[sjaensch]

I wouldn't worry too much about the coverage check, it's ok for it to go down a bit with changes like these. Looking at the latest changes, I feel you're trying to do too much to test what essentially is a documented feature of PyYAML. I really think we can go ahead with just the changes to bravado_core/spec.py.

[hiromu]

As you won't mind the degradation of the coverage rate, I removed the newly added test condition. I think it now ready to be merged.

[hiromu]

After hours of investigation, I found there are several issues in pip and pyyaml which we cannot easily address:

• Building pyyaml from its source with Python2.7 shows incompatibility in the environment on Travis due to the version of setuptools.
• The current version of pip does not support specifying both --global-option and python_version simultaneously.

Thus, I decided to explicitly skip building pyyaml in the case of Python 2.7. I believe this is a reasonable way under these restrictions.

[sjaensch]

Thanks for your contribution @hiromu!