fix: As of django-filer 3.0 mptt is not needed as a dependency

[fsbraun]

Description

Describe:

• This PR only automatically adds mptt to INSTALLED_APPS for django-filer, if the installed version of django-filer is below 3.0.
• django-filer as of version 3.0 does not depend on mptt any more.
• Without this change, a ModuleNotFound exception will be raised in an environment without mptt

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [ ] changes file included (see docs)
• [ ] Usage documentation added in case of new features
• [ ] Tests added

[codecov[bot]]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.24 :warning:

Comparison is base (1bba8eb) 92.68% compared to head (264b1bd) 92.45%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #224 +/- ## =========================================== - Coverage 92.68% 92.45% -0.24% =========================================== Files 9 9 Lines 793 795 +2 Branches 124 125 +1 =========================================== Hits 735 735 - Misses 22 23 +1 - Partials 36 37 +1 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `92.45% <33.33%> (-0.24%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/nephila/django-app-helper/pull/224?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila) | Coverage Δ | | |---|---|---| | [app\_helper/utils.py](https://app.codecov.io/gh/nephila/django-app-helper/pull/224?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila#diff-YXBwX2hlbHBlci91dGlscy5weQ==) | `92.97% <33.33%> (-0.78%)` | :arrow_down: |

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[yakky]

@fsbraun thanks, I merged it with #226 and released version 3.3.1