Fix issue 12 and extend CI tests to additional Python versions/OSes

[lbianchi-lbl]

Fixes #12

Proposed changes:

• Add an explicit Python version check to exclude incompatible versions from workaround
• Extend CI tests so that they run on Python version/OS combination matrix

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[dangunter]

Looks good

[andrewlee94]

Rubber stamp.

[lbianchi-lbl]

@dangunter not sure what's the status for Python 3.9 but we can try to see how it behaves if we remove the workaround once we start supporting 3.9.

In the meanwhile, what I'm getting from tornadoweb/tornado#2608 is that, for the time being, this should be dealt with on the side of the application: so our workaround is the recommended way to address this. In any case he issue looks quite active and we can use it as a reference to see if there's any progress.

I've added this information as a comment in the code, so if you're OK with the wording you can go ahead and re-approve it.

[andrewlee94]

Looks good to me.

[ksbeattie]

LGTM