Fix deepcopy issue on Python3.5.0

[macisamuele]

While working on Yelp/bravado#445, I've noticed that the deepcopy support added in #360 seems to not be working on Python3.5.0.

The goal of this PR is to fix the issue.

NOTE: This PR is still in draft as the issue is not solved, but as least we can reproduce it via travis

[coveralls]

Coverage decreased (-0.1%) to 98.395% when pulling 59f0a36 on macisamuele:maci-run-tests-on-python-3.5.0 into 9c20a25 on Yelp:master.

[macisamuele]

I've dug a bit more into the test failures in Python 3.5.0 and I did found out that the issue is a known issue for the CPython implementation (issue link) and that it has already been resolved.

Unfortunately, the RefResolver of jsonschema library does use lru_cache into the class attributes and so this triggers the issue.

I've not found an easy/nice way to work this around but at the same time I do not expect that a lot of our users are using Python 3.5.0 (which should be the only affected version

My proposal at this point would be to update the bravado-core changelog highlighting the fact that deepcopy might not work properly on CPython 3.5.0.

Test run on all versions is available at: https://gist.github.com/macisamuele/edbdfee3bfdd10c33cc40ab654d9cb71

[sjaensch]

I do not think this is a good pattern. If some part of our software is known to not work with an older Python version then either we need a workaround, disable that feature or discontinue support for that Python version.

I vote for the latter. There are three major Python releases since 3.5 out now, plus we don't need to drop support for all of Python 3.5, just 3.5.0. Other libraries do this as well, e.g. aiohttp requires Python 3.5.3+.

[macisamuele]

Agreed.
Going to drop this PR in favour of #366