Require enum34 only for Python versions before 3.4

[ts826848]

enum34 appears to be a potential source of problems when installed in
environments using Python 3.4 or later [0, 1]. Apparently enum34
1.1.8 was able to avoid these issues, but a regression was introduced in
1.1.9 and is still present in the most recent release [2].

This commit loosens the requirement on enum34 so that it is only
installed if Python 3.3 or older is used.

This particular solution requires setuptools v36.2 or newer [3]. If
older versions of setuptools need to be supported, then the requirements
can be put into an extras_require clause [4, 5].

[0]: python-poetry/poetry#1122
[1]: iterative/dvc#1995
[2]: https://web.archive.org/web/20200621215937/https://bitbucket.org/stoneleaf/enum34/issues/31/make-the-install-a-noop-on-python36
[3]: https://setuptools.readthedocs.io/en/latest/history.html#v36-2-0
[4]: robshakir/pyangbind#232 (comment)
[5]: https://gitlab.com/pycqa/flake8/-/merge_requests/191/diffs

[ghost]

All CLA requirements met.

[ts826848]

In my specific case, I ran into issues compiling a project with Nuitka when mssql-cli was installed in the same virtualenv.