Add support for Python 3.9, drop EOL 3.5

[hugovk]

Here's the pip installs for p_tqdm from PyPI for December 2020:

category	percent	downloads
3.6	48.25%	1,598
3.7	37.71%	1,249
3.8	10.24%	339
null	2.63%	87
3.9	0.72%	24
2.7	0.24%	8
3.5	0.21%	7
Total		3,312

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

Also:

• Declare support for and test Python 3.9, released October 2020
• Remove unused six dependency
• Add python_requires to help pip
• Upgrade Python syntax with https://github.com/asottile/pyupgrade --py36-plus

[hugovk]

✅ Passing CI build: https://travis-ci.org/github/swansonk14/p_tqdm/builds/753153139

[hugovk]

Closing this year-old PR, but happy to re-open if you like! We can now add 3.10 and drop 3.6.