housekeep.py: Fix possible TypeError

[loqs]

What do these changes do?

Prevent a possible TypeError when running housekeep.py gather outside of tox by returning if TOX_ENV_NAME is not set.

Are there changes in behavior for the user?

No.

Related issue number

Fixes #597.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): qa
  • Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• Documentation reflects the changes
• Add a news fragment into the NEWS.rst file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.81%. Comparing base (dc78ef6) to head (9f359e8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
+ Coverage   97.75%   97.81%   +0.05%     
==========================================
  Files          23       23              
  Lines        5709     5709              
  Branches      351      351              
==========================================
+ Hits         5581     5584       +3     
+ Misses         79       75       -4     
- Partials       49       50       +1     

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