Add type hints and pre-commit hooks (black+isort+mypy)

[eggplants]

Closes: #71, Ref: https://github.com/gweis/isodate/pull/66#issuecomment-992006703

---

$ pre-commit run --all
pyupgrade................................................................Passed
flake8...................................................................Passed
check blanket noqa.......................................................Passed
Check for merge conflicts................................................Passed
Check Yaml...............................................................Passed
Fix End of Files.........................................................Passed
Trim Trailing Whitespace.................................................Passed
black....................................................................Passed
isort....................................................................Passed
mypy.....................................................................Passed

[eggplants]

$ git rev-parse --short HEAD
736238b
$ pytest
==================================================== test session starts =====================================================
platform darwin -- Python 3.9.9, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: /Users/eggplants/prog/isodate, configfile: setup.cfg
plugins: subtests-0.5.0, monkeytype-1.1.0, mock-3.7.0, typeguard-2.13.3, cov-3.0.0
collected 18 items

src/isodate/tests/test_date.py .                                                                                       [  5%]
src/isodate/tests/test_datetime.py .                                                                                   [ 11%]
src/isodate/tests/test_duration.py ..........                                                                          [ 66%]
src/isodate/tests/test_pickle.py ....                                                                                  [ 88%]
src/isodate/tests/test_strf.py .                                                                                       [ 94%]
src/isodate/tests/test_time.py .                                                                                       [100%]

===================================================== 18 passed in 0.13s =====================================================
$ pre-commit run --all
pyupgrade................................................................Passed
flake8...................................................................Passed
check blanket noqa.......................................................Passed
Check for merge conflicts................................................Passed
Check Yaml...............................................................Passed
Fix End of Files.........................................................Passed
Trim Trailing Whitespace.................................................Passed
black....................................................................Passed
isort....................................................................Passed
mypy.....................................................................Passed

[eggplants]

@gweis @hugovk ready for review.

[eggplants]

@hugovk done!

[eggplants]

@hugovk done

[eggplants]

@gweis How are you?

[eggplants]

@gweis ping