fix `pytest_ignore_collect` not to block default pytest code

[mgorny]

Fix pytest_ignore_collect hook implementation to return None rather than False when the path ought not to be ignored. This is necessary to enable the default pytest implementation of pytest_ignore_collect() to be used. Otherwise, the default rules are never applied and e.g. --ignore does not work.

I've also reported https://github.com/pytest-dev/pytest/issues/12383 about the misleading documentation.

[coveralls]

coverage: 98.526%. remained the same when pulling 2a8eaf372acfca7c60a540f1856c3f28a655ebed on mgorny:pytest-ignore into dccae40a247f09a9e1b0f192c295a30786878d5a on pytransitions:master.

[aleneum]

Hi @mgorny,

thanks a lot for that fix! Good to know that pytest_ignore_collect expects True or None as return value.

Cheers!

[mgorny]

Thanks!