CI: Fail when open()’s default encoding is used

The previous few commits have removed all calls to open() that use its
default encoding. That being said, it’s still possible that code added
in the future will contain that same mistake. This commit makes it so
that the CI test job will fail if that mistake is made again.

Unfortunately, it doesn’t look like coverage.py allows you to specify -X
options [1] or warning filters [2] when running your tests [3]. As a
result, the CI test job will also fail if coverage.py uses open()’s
default encoding. Hopefully, coverage.py won’t do that. If it does, then
we can always temporarily revert this commit.

[1]: <https://docs.python.org/3.12/using/cmdline.html#cmdoption-X>
[2]: <https://docs.python.org/3.12/using/cmdline.html#cmdoption-W>
[3]: <https://coverage.readthedocs.io/en/7.4.0/cmd.html#execution-coverage-run>

.github/workflows/ci.yaml

@@ -56,6 +56,13 @@ jobs:
       - run: pip install .
       # https://github.com/AndreMiras/coveralls-python-action/issues/18
       - run: echo -e "[run]\nrelative_files = True" > .coveragerc
-      - run: coverage run -m unittest discover
+      - run: >-
+          python
+          -X warn_default_encoding
+          -W error::EncodingWarning
+          -m coverage
+          run
+          -m unittest
+          discover
       - name: Coveralls
         uses: AndreMiras/coveralls-python-action@develop