Changed paths to work with build tools

[MaximilianSoerenPollak]

This is the second 'split' PR that was discussed for splitting the big one into smaller ones. This one here #80

This PR just fixes the pathing in the library, making it work with other systems that expect a more modern approach than the one with setup tools (which is not needed anymore since python 3.3) that was used.
Setuptools was a necessary change in this PR as otherwise the tests will not work, due to failing pip install .

fixes #79

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**Note: I need some help on this PR, as I run into a small problem. I uploaded the noxtest.py as well so you can see how I tested this change. **

When I run nox with this the outcome is as follows:

nox > Session tests-3.12(sphinx='7.4.1') was successful.
nox > Running session lint
nox > Missing interpreters will error by default on CI systems.
nox > Session lint skipped: Python interpreter 3.9 not found.
nox > Running session linkcheck
nox > Missing interpreters will error by default on CI systems.
nox > Session linkcheck skipped: Python interpreter 3.9 not found.
nox > Ran multiple sessions:
nox > * tests-3.10(sphinx='7.1.2'): failed
nox > * tests-3.10(sphinx='7.2.5'): failed
nox > * tests-3.10(sphinx='7.4.1'): success
nox > * tests-3.11(sphinx='7.1.2'): skipped
nox > * tests-3.11(sphinx='7.2.5'): skipped
nox > * tests-3.11(sphinx='7.4.1'): skipped
nox > * tests-3.12(sphinx='7.1.2'): failed
nox > * tests-3.12(sphinx='7.2.5'): failed
nox > * tests-3.12(sphinx='7.4.1'): success
nox > * lint: skipped
nox > * linkcheck: skipped

For Sphinx 7.4.1 it works fine and has no issues but the other two versions fail. With this error:

FAILED tests/test_custom_template.py::test_custom_utf8_template[test_app0] - sphinx.errors.ThemeError: An error happened in rendering the page index.
FAILED tests/test_custom_template.py::test_custom_koi8_template[test_app0] - sphinx.errors.ThemeError: An error happened in rendering the page index.
FAILED tests/test_custom_template.py::test_custom_template[test_app0] - sphinx.errors.ThemeError: An error happened in rendering the page index.

Some more context:

E           sphinx.errors.ThemeError: An error happened in rendering the page index.
E           Reason: TemplateNotFound("'about.html' not found in ['/tmp/tmpxvaxwngv/custom_tr_template/_templates', '/home/maxi/tmp/testing/sphinx-test-reports/.nox/tests-3-12-sphinx-7-2-5/lib/python3.12/site-packages/sphinx/themes/basic']")

This always seems to stem from the tests/test_custom_template.py file. All of the app.build() calls there fail due to this same error.

I'm not sure how to fix this as I can't find the 'about.html' anywhere in the project?

Let me know if you have any more insight or need some more information from my side to triage this.