Add support for python 3.7

[ndparker]

Hi!

Python 3.7 raises the following warning:

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

We're running our tests with all warnings turned to errors, so they fail on that.
This PR fixes that (and adds python 3.7 to tox.ini).

[HelloThisIsFlo]

Hey,

Great job @ndparker I came here specifically to see if there was a fix for this in the works, glad to see you already pushed a pull request.

It seems the only reason the build failed on Travis was on python 3.3. It didn't seem related to the changes. I investigated, and found this: aws/base64io-python#4 (comment)

If you could add that small fix to the .travis.yml that would be great:

if [[ $TRAVIS_PYTHON_VERSION == 3.3 ]]; then pip install virtualenv==15.2.0 tox==3.1.3; fi

So that would mean replacing:

install: pip install tox-travis

with

install:
  - if [[ $TRAVIS_PYTHON_VERSION == 3.3 ]]; then pip install virtualenv==15.2.0; fi 
  - pip install tox-travis

Once that's done, @Xion could you give it a look and merge it? Python 3.8 will be released around october and callee will stop working with that new version. It'd be huge bummer for the project.

Either way, Thanks both for your great work! 💯🎉😃

[untidy-hair]

@Xion It would be great to see callee work with 3.7 and 3.8! thanks

[fperetti]

@Xion @untidy-hair created this PR #13 with the suggested changes.

[Xion]

Sorry, I had been rather busy in RL and didn't have a chance to look at the PR until now :( I merged it though, and released 0.3.1 which incorporates it.

I will also verify whether the lib works with py3.8 and add it to the supported versions.