dpkg cannot cleanup pytest pycache on purge

[joshschmelzle]

When wlanpi-hwtest is removed via apt purge wlanpi-hwtest, we receive some dpkg warnings about pycache folders for pytest not being empty.

This is low priority as it does not appear to impact anything besides leaving some extra folders on the host when we purge the Debian package.

dpkg: warning: while removing wlanpi-hwtest, directory '/opt/wlanpi-hwtest/lib/python3.7/site-packages/pytest_metadata/ci/__pycache__' not empty so not removed
dpkg: warning: while removing wlanpi-hwtest, directory '/opt/wlanpi-hwtest/lib/python3.7/site-packages/pytest_metadata/__pycache__' not empty so not removed
dpkg: warning: while removing wlanpi-hwtest, directory '/opt/wlanpi-hwtest/lib/python3.7/site-packages/pytest_html/__pycache__' not empty so not removed
dpkg: warning: while removing wlanpi-hwtest, directory '/opt/wlanpi-hwtest/lib/python3.7/site-packages/hwtest/tests/__pycache__' not empty so not removed 

[joshschmelzle]

References:

• https://docs.pytest.org/en/stable/cache.html#clearing-cache-content
• https://docs.pytest.org/en/stable/assert.html?highlight=dont_write_bytecode#assertion-rewriting-caches-files-on-disk