Make --strict imply both strictness options

[The-Compiler]

RFC for #7503. It looks like there was a consensus that this is the right thing to do, but not on whether this can still go to pytest 6 in such a late stage or should wait for pytest 7.

IMHO it's fine to have in pytest 6 since we already documented this possibility in the changelog for pytest 4.5.0 ("The existing --strict option has the same behavior currently, but can be augmented in the future for additional checks.") and discouraged it in the documentation (The use of --strict-markers is highly preferred. --strict was kept for backward compatibility only and may be confusing for others as it only applies to markers and not to other options.)

Let me know what you think.

[The-Compiler]

Looks like we're not reaching a consensus here in #7503, and it'd be better to deprecate --strict for 6.0 and then take care to get it right in 7.0. Thus I'm closing this.