Drop support for EOL Python

[hugovk]

Python 2.6-3.5 are EOL and no longer supported by the upstream CPython team.

They're also little used. Here's the pip installs for pyperclip from PyPI for August 2021:

category	percent	downloads
3.7	39.55%	868,110
3.8	24.16%	530,407
3.6	15.43%	338,797
null	8.92%	195,714
3.9	8.54%	187,457
2.7	3.02%	66,251
3.5	0.29%	6,398
3.4	0.05%	1,174
3.10	0.04%	773
3.11	0.00%	5
3.3	0.00%	5
Total		2,195,091

Source: pip install -U pypistats && pypistats python_minor pyperclip --last-month

Dropping allows to use modern Python features and reduces the maintenance burden.

• Also upgrade to modern Python syntax using https://github.com/asottile/pyupgrade --py36-plus.

• And add python_requires to setup to help pip install the best version for the user's running Python.

This renders obsolete some old issues/PRs:

• Closes Doesn't work with Python 2 on KDE #133
• Closes pyperclip exposes pygtk bug in Python 2 on Linux #101
• Closes Build universal wheels #131
• Closes Declare support for Python 3.9 #187
• Includes and closes Add CI: test on GitHub Actions #211

[asweigart]

I understand your argument, but the maintenance burden for having Pyperclip continue to run on Python 2.7 is minimal. The entire code base is under 1000 lines long. I don't really see a need to drop support for it. Thank you.

[hugovk]

the maintenance burden for having Pyperclip continue to run on Python 2.7 is minimal.

Okay!

How about 2.6 and 3.1, 3.2, 3.3, 3.4, 3.5?

I'll make a new PR that just adds Python 3.10 support and python_requires.