Make conditional enum34 dependency work with earlier setuptools versions.
Make conditional enum34 dependency work with earlier setuptools versions.

A few recent OS distros (RHEL 7, CentOS, both ~ year old) use _very_ old
setuptools versions (e.g. 0.9.8), which don't support the "environment marker"
(e.g., "depname; python_version < X") install_requires syntax, which was
introduced in setuptools 36.2.

While the best fix is for users to use more recent setuptools versions, that
isn't always possible or very inconvenient. While building wheels might be
another solution, it would be a larger change to more pieces I'm even less
familiar with.

To fix, manually compute the dependency requirement when running under
earlier setuptools versions. This works because we use sdist, which means
setup.py is run directly on the host installing absl.

Fixes #79

Manually tested on the following (see issue for details):
 * RHEL 7.4 (Google Cloud), Python 2.7.5, pip 8.1.2, setuptools 0.9.8
 * CentOS 7 (Google Cloud), Python 2.7.5, pip 8.1.2, setuptools 0.9.8
 * Debian/Ubuntu, Python 3.6.6, pip 18.1, setuptools 40.4.3
 * Debian/Ubuntu, Python 2.7.15, pip 18.1, setuptools 0.9.8
 * Debian/Ubuntu, Python 2.7.15, pip 18.1, setuptools 40.4.3

PiperOrigin-RevId: 218904912

abseil/abseil-py,abseil/abseil-py