do not imply pre-release when `!=` operator is used

[Czaki]

Summary

closes #6640

Test Plan

Could you suggest how I should test it?

(already tested locally)

[charliermarsh]

This looks right to me! It will need to be part of 0.5.0 so I'll add it to that milestone.

[charliermarsh]

I think you could write a test like prerelease_upper_bound_exclude... flask<2.0.1, !=2.0.0rc1 and ensure that 2.0.0rc2 is not selected.

[zanieb]

@Czaki it looks like this has some unrelated changes in it now.

[charliermarsh]

I backed them out. @zanieb, want to merge into 0.5 branch?