Fix Altair import in tools scripts

[binste]

The current importing behaviour in the tools scripts has some inconsistencies which this PR tries to address. These issues were partially discovered in #2814.

Issue 1: Altair is imported for generate_api_docs before the vegalite files are updated

The statement import generate_api_docs was previously at the top of generate_schema_wrapper.py. In generate_api_docs.py, one of the first statements is the import of Altair -> Altair got imported before generate_schema_wrapper can update the vegalite files. This was presumably not a big issue so far as this script is usually run multiple times anyway and so the api docs were updated eventually.

This PR solves this by moving the import generate_api_docs statement at the end of the main function in generate_schema_wrapper.py.

Issue 2: If Altair is installed as a site-package, it might be imported instead of the currently checked out version

If you execute python tools/generate_api_docs.py then Python correctly uses Altair from the root folder due to the modification of sys.path in the beginning of the script. However, if generate_api_docs is improted from generate_schema_wrapper, Python seems to use sys.path as it's defined in generate_schema_wrapper for the whole import procedure, i.e. sys.path needs to be modified to be able to import Altair before generate_api_docs is imported.

You can check this by installing Altair in site-packages with pip and then adding the following lines in generate_api_docs.py right after the Altair import:

print("--------")
print(alt.__file__)
print("--------")

On the current master branch, when I execute python tools/generate_schema_wrapper.py I get the path to my site-packages Altair:

--------
/home/vscode/.local/lib/python3.11/site-packages/altair/__init__.py
--------

When I execute python tools/generate_api_docs.py I get the path to the correct local version in the current folder (/workspaces/altair):

--------
/workspaces/altair/altair/__init__.py
--------

This PR solves this by adding the root folder to sys.path already in generate_schema_wrapper.py so it doesn't matter anymore if you call python tools/generate_schema_wrapper.py or python tools/generate_api_docs.py. Both now import the correct local version of Altair.

[mattijn]

I've test this PR and can confirm it is now working correctly. I've been wondering before why the api docs were different when generated from within the generate_schema_wrapper.py vs generating it directly using generate_api_docs.py. Merging. Thanks @binste!