ENH: Consider warnings as errors when running pytest

[jhlegarreta]

Consider warnings as errors when running pytest: add the strict-warnings flag to pytest to consider warnings as errors.

[oesteban]

pytest: error: unrecognized arguments: --strict-warnings
  inifile: /src/mriqc/pyproject.toml
  rootdir: /src/mriqc

[jhlegarreta]

Yes, there is efforts across other NiPreps repositories to enforce this as well, e.g.
nipreps/nireports#157
nipreps/nifreeze#40

Only the latter seems to be working. There is a discussion with Chris on the nireports PR. It may be worthwhile going over this during the next tech meeting to avoid accumulating technical debt.

PS: Unrelated to the topic and suitable for a tech meeting: should the default branch be renamed to main for the sake of consistency with modern practices/other NiPreps repositories?