Add code coverage

[pelletier]

Set up make coverage to report coverage, and fixed it to 100% on top of the #12 branch. Far from perfect, but helped me catch some edge cases I didn't test.

It uses coverage.py, which has a nice HTML interface:

As well as an in-console report:

(dispatch-sdk) ~/s/g/s/dispatch-sdk-python cov$ make coverage
python -m mypy src tests
Success: no issues found in 55 source files
coverage run -m unittest discover
....................................
----------------------------------------------------------------------
Ran 36 tests in 0.112s

OK
coverage html -d .coverage-html
Wrote HTML report to .coverage-html/index.html
coverage report
Name                                Stmts   Miss  Cover
-------------------------------------------------------
src/dispatch/__init__.py               74      0   100%
src/dispatch/coroutine.py             163      0   100%
src/dispatch/fastapi.py                62      0   100%
src/ring/__init__.py                    0      0   100%
src/ring/coroutine/__init__.py          0      0   100%
src/ring/coroutine/v1/__init__.py       0      0   100%
src/ring/record/__init__.py             0      0   100%
src/ring/record/v1/__init__.py          0      0   100%
src/ring/status/__init__.py             0      0   100%
src/ring/status/v1/__init__.py          0      0   100%
src/ring/task/__init__.py               0      0   100%
src/ring/task/v1/__init__.py            0      0   100%
-------------------------------------------------------
TOTAL                                 299      0   100%

It's not hooked up to CI. Just using it as a guidance tool.