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.

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Update rPGP to 0.10.1. #4236
 
 ### Fixes
+- Fix python bindings README documentation on installing the bindings from source.
 
 ## [1.112.3] - 2023-03-30