- Update the development-related dependencies.

- Demote Python v3.6 and v3.7 to minimally supported versions (no
linting, no coverage).

- Fix https://github.com/OpenCyphal/pydsdl/issues/84. `os.path` is no
longer used, and dependency on the text representations of paths is
minimized.

- Refactor the test suite to eliminate the global state and simplify the
internal APIs.

- The library now logs a warning when it encounters a `*.uavcan` file,
suggesting renaming it to `*.dsdl`. See
https://forum.opencyphal.org/t/uavcan-file-extension/438/8. There are no
plans to remove support for `*.uavcan` but it is desirable to push users
towards consistency.

- Bump the minor version.