Allow using Cobertura XML report in incremental mode

I'm trying to use diff-cover with mypy to enforce any new code to have type annotations. Here's what I use (via tox):

mypy --cobertura-xml-report {envtmpdir} qutebrowser tests {posargs}
diff-cover --fail-under=100 --compare-branch={env:DIFF_BRANCH:origin/{env:GITHUB_BASE_REF:master}} {envtmpdir}/cobertura.xml

That does seem to work fine - however, due to #6076 (cc @mthuurne), it disables incremental mode (causing the runtime of mypy to go up from <1s to ~15s). Note that in my case I don't care about the XML report only containing changed files, because those are the only ones actually relevant for the annotation coverage diff - thus, I think it would be fine to enable the cache for this use-case.

I understand if this is a too specific case, though - I wonder if it'd make sense to have some "enforce annotations for any new code" functionality in mypy itself (perhaps via the incremental mode)?

Inizia tracciando il punto di ingresso di mypy per --cobertura-xml-report e il comportamento della modalità incrementale descritto in issue #6076. Verifica l'esempio di tox con mypy e diff-cover e considera completato il lavoro quando la generazione del report XML Cobertura non disabilita più la modalità incrementale per questo caso d'uso.