fix: manually update pip-tools version to 6.5.1 to fix pip/pip-tools bug

The following comment is borrowed from this pull request, with changes: openedx/opaque-keys#218

This failure started happening in the automated requirements upgrade GitHub action:
https://github.com/openedx/edx-submissions/runs/5704384647?check_suite_focus=true

The bug was being caused by an interaction between pip and pip-tools, described here:
jazzband/pip-tools#1558

The bug is fixed in pip-tools==6.5.0 - from this PR:
jazzband/pip-tools#1567

This pull requests bumps the version of pip-tools manually to get the automated upgrades working again.

requirements/pip-tools.txt

@@ -8,7 +8,7 @@ click==8.0.3
     # via pip-tools
 pep517==0.12.0
     # via pip-tools
-pip-tools==6.4.0
+pip-tools==6.5.1
     # via -r requirements/pip-tools.in
 tomli==2.0.0
     # via pep517