Remove install_python_bindings.py script and update docs

`install_python_bindings.py` was not used by CI
and scripts, except for `scripts/run-python-test.sh`
which only used it to invoke `cargo`.

Instead of using an additional script,
run cargo directly.

The documentation is updated to remove
references to `install_python_bindings.py`.
The section "Installing bindings from source"
was in fact incorrect as it suggested
running `tox --devenv` first and only then
compiling the library with `install_python_bindings.py`.
Now it explicitly says to run cargo first
and then install the package without using `tox`.
`tox` is still used for running the tests
and setting up development environment.

python/tox.ini

@@ -43,7 +43,7 @@ deps =
     pygments
     restructuredtext_lint 
 commands =
-    black --quiet --check --diff setup.py install_python_bindings.py src/deltachat examples/ tests/
+    black --quiet --check --diff setup.py src/deltachat examples/ tests/
     ruff src/deltachat tests/ examples/
     rst-lint --encoding 'utf-8' README.rst