Python: fix debian cleanup scripting

What does this MR do?

Fix cleanup calls in Debian Dockerfile for Python.

Typo of python.3.8 in place of python3.8.

See https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3113#note_1172951679

Compare:

• https://gitlab.com/gitlab-org/build/CNG/-/blob/15-4-stable/gitlab-python/Dockerfile#L29
• https://gitlab.com/gitlab-org/build/CNG/-/blob/15-4-stable/gitlab-python/Dockerfile.build.ubi8#L20

Related issues

Related to https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3113

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated