Drop support for EOL Python 2.7 and 3.5

[hugovk]

Python 2.7 is EOL since 2020-01-01.

Python 3.5 is EOL since 2020-09-30.

After prettytable 1.0 has been released (#18), support for Python 2.7 and 3.5 will be dropped, and released as prettytable 2.0.0.

Prettytable 1.x will be the last to support Python 2.7 and 3.5, and will remain available on PyPI.

The python_requires metadata will help those with modern pip (v9+) get the newest version of prettytable that supports their Python version.

[parth-verma]

Is this issue up for grabs?

[hugovk]

Not yet, still pending the release of prettytable 1.0 (#18), which will be the last to support 2.7.

But thanks for asking!

[hugovk]

@parth-verma PrettyTable 1.0 is almost out the door, would you like to take this issue?

We can also drop 3.5 too.

A few things to do:

• Update Trove classifiers
• Add Programming Language :: Python :: 3 :: Only classifier
• Update python_requires
• Update tox.ini
• Remove from CI
• Update --target-version in .pre-commit-config.yaml

Also run pip install -U pyupgrade && pyupgrade **/*.py --py36-plus and add args: ["--py36-plus"] to pyupgrade in .pre-commit-config.yaml

[hugovk]

Please see PR #68.