Dropped support for Python 2

[tgaddair]

Fixes #1929.

• Removed Python 2 tests from Buildkite.
• Removed references to Python 2 in Docker images.
• Removed system version checks that perform different logic for Python 2 vs Python 3.
• Removed dependency on six and replaced all usages with native Python 3 equivalents.
• Removed doc references to Python 2 and added Python 3 to install requirements.
• Removed unit test exclusions for Python 2.
• Removed Python 2 from supported versions in setup.

[EnricoMi]

Do we need to mention Python3 in docs somewhere?

[tgaddair]

Do we need to mention Python3 in docs somewhere?

I added a mention in the Install Guide under "requirements". Might be worth it to put it somewhere on the main README, but couldn't think of a good place.

[EnricoMi]

Do we need to mention Python3 in docs somewhere?

I added a mention in the Install Guide under "requirements". Might be worth it to put it somewhere on the main README, but couldn't think of a good place.

I think install.rst is good enough, haven't seen that before I send that comment.

[EnricoMi]

Are these imports due to Python 2 compatibility?

try:
    from shlex import quote
except ImportError:
    from pipes import quote

try:
    from collections.abc import Iterable
except ImportError:
    from collections import Iterable

[tgaddair]

Are these imports due to Python 2 compatibility?

try:
    from shlex import quote
except ImportError:
    from pipes import quote

try:
    from collections.abc import Iterable
except ImportError:
    from collections import Iterable

Good catch, looks like these imports both work in Python 3.6 and above, so I'll update.

[EnricoMi]

There is one more Python2 condition in .buildkite/gen-pipeline.sh: https://github.com/horovod/horovod/blob/master/.buildkite/gen-pipeline.sh#L261. Created #1958 to fix that and some lost integration tests.