Default expansion is more strict now (undefined vs. empty).
The supported set is closer to what is available in docker-compose
See: https://docs.docker.com/compose/environment-variables/env-file/

Single quoted strings do not expand variables in bash.
I think the libary was missing a simple way
to selectively disable variable expansion.

Error results in no quote.

I corrected flake8 errors in this commit.
test_parser.py had a lot of long lines, formatted it with black.

Importing sh is done in try/except. Although the package
is available for Windows, it is failing on import.

Moved previous manual tests into proper pytest.

Utility for simulating Windows os.environment behavior
moved to a separate file.

This commit brings back the old interface for `resolve_variables`.
I realized it might be actually used by the clients.

I made _resolve_bindings private.
Created resolve_variable as a result of refactoring.
It be as useful as resolve_variables for the clients.

With an option to have a different behavior for single quoted
strings it would be nice to use mixed string (e.g. "ffo"'${bar}'
to produce "foo${bar}".

Intruduced it by reading multiple strings on the value side.
Unquoted strings are tricky because of the whitespace and comments.
All the rest should come for free, as it is a change from
handling a single value to handling a list of values and concatenation.

There was actually one bug detected by the typing check.
Value without '=' did not work properly. Added a test for that.

Add windows to tested operating systems

Addresses non-intuitive behavior of `dotenv run python ...` on Windows