Drop Python 3.6 and add some type hints

[hugovk]

We need to drop 3.6 to use from __future__ import annotations. Could use 3.6 compatible things like from typing import Tuple, Union, but 3.6 is soon EOL:

cycle	latest	release	eol
3.10	3.10.0	2021-10-04	2026-10-04
3.9	3.9.7	2020-10-05	2025-10-05
3.8	3.8.12	2019-10-14	2024-10-14
3.7	3.7.12	2018-06-27	2023-06-27
3.6	3.6.15	2016-12-23	2021-12-23

And has lowest of the (quite low anyway) pip installs from PyPI for September 2021:

category	percent	downloads
3.8	35.78%	307
3.7	23.43%	201
null	20.40%	175
3.9	12.94%	111
3.6	7.46%	64
Total		858

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

Plus refactor some tests to use pytest.mark.parametrize and add pytest.param(..., id=...) to make nicer verbose output.

[codecov]

Codecov Report

Merging #282 (83eadc0) into main (13b1e52) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
- Coverage   99.60%   99.59%   -0.01%     
==========================================
  Files           8        8              
  Lines         754      740      -14     
==========================================
- Hits          751      737      -14     
  Misses          3        3              

Flag	Coverage Δ
GHA_Ubuntu	99.59% <100.00%> (-0.01%)	⬇️
GHA_Windows	99.59% <100.00%> (-0.01%)	⬇️
GHA_macOS	99.59% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pypistats/__init__.py	99.26% <100.00%> (+<0.01%)	⬆️
src/pypistats/cli.py	98.80% <100.00%> (+0.01%)	⬆️
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_pypistats.py	100.00% <100.00%> (ø)
tests/test_pypistats_cache.py	100.00% <100.00%> (ø)
tests/test_pypistats_print.py	100.00% <100.00%> (ø)

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[hugovk]

Coverage percentage dropped because (covered) lines were removed in the refactoring. The number of missed lines (3) didn't change.