Replace pkg_resources with importlib_resources

[foarsitter]

In preparation to support Python 3.12 pkg_resources need to be removed.

According to there migration guide it can be replaced by importlib_resources

https://importlib-resources.readthedocs.io/en/latest/migration.html#pkg-resources-resource-filename

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (289de36) 96.45% compared to head (8eee2ad) 96.45%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #562 +/- ## ======================================= Coverage 96.45% 96.45% ======================================= Files 20 20 Lines 2733 2733 ======================================= Hits 2636 2636 Misses 97 97 ``` | [Files Changed](https://app.codecov.io/gh/python-restx/flask-restx/pull/562?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-restx) | Coverage Δ | | |---|---|---| | [flask\_restx/schemas/\_\_init\_\_.py](https://app.codecov.io/gh/python-restx/flask-restx/pull/562?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-restx#diff-Zmxhc2tfcmVzdHgvc2NoZW1hcy9fX2luaXRfXy5weQ==) | `82.14% <100.00%> (ø)` | |

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