fix: Update manifest to exclude mptt stub in release package

[fsbraun]

Description

The repo contains an empty mptt stub since app-helper automatically adds it to INSTALLED_APPS for test runs. Tests would fail w/o a (potentially empty) mptt package.

The manifest now excludes the stub from the filer package to not interfere with installations that need django-mptt for other reasons.

The mptt stub can be removed from the django-filer repo once app-helper is updated.

Related resources

• fix: As of django-filer 3.0 mptt is not needed as a dependency nephila/django-app-helper#224
• #...

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #1367 (ef520b6) into master (0f679cd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1367   +/-   ##
=======================================
  Coverage   73.65%   73.65%           
=======================================
  Files          75       75           
  Lines        3408     3408           
  Branches      551      551           
=======================================
  Hits         2510     2510           
  Misses        727      727           
  Partials      171      171           

Impacted Files	Coverage Δ
filer/models/abstract.py	81.60% <100.00%> (ø)