py-tox: updated to 4.15.0

Features - 4.15.0
Add support for multiple appending override options (-x, 窶登verride) on command line
Add support for inverting exit code success criteria using bang (!)

Bugfixes - 4.15.0
Fix issue that the leading character c was dropped from packages in constraints files
Allow appending to deps with --override testenv.deps+=foo
Fix non-existing branch rewrite in the documentation to main.
Update test typing for build 1.2.0, which has an explicit Distribution type
Fix broken input parsing for --discover flag.

Improved Documentation - 4.15.0
Rephrase --discover flag窶冱 description to avoid confusion between paths and executables.

(adam)