Fixing enum34 requirement for python versions <= 3.4 in setup.py

[alexhagen]

Was running into error on Python3.6 and pip 18.1

error in absl-py setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers; Expected version spec in enum34;python_version<='3.4' at ;python_version<='3.4'

Using the install_requires and extra_requires fixed my problems, and is ostensibly a better way to do this see reference here.

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[alexhagen]

I signed it!

[googlebot]

CLAs look good, thanks!

[rickeylev]

Hi Alex,

Thanks for the PR, but I'm prepping a different fix.

btw, this is being trackd in #79
Please chime in with your OS, Python version, pip version, and setuptools version (see issue for commands on how to get these) if it isn't already listed there.

[rickeylev]

Oh, and that bug also has a link to the upcoming fix that you can try; if it doesn't work for you, please chime in on the bug to let me know.

[rickeylev]

Closing, since the other commit fixed this.