More unit tests for settings.py, specifically syntax and indentation error reporting of pelicanconf.py

[egberts]

• I have read the Filing Issues and subsequent “How to Get Help” sections of the documentation.
• I have searched the issues (including closed ones) and believe that this is not a duplicate.

Issue

Added several more unit tests of SyntaxError reporting and IndentationError of pelicanconf.py into a separate test_settings_syntax.py.

Only one-line of non-test code were changed to accommodate initial deployment of pytest in pelican/__init__.py: pytest makes repeated use of rescanning the Python code and __path__ is undefined during the 1st pass, hence new logic:

if "__path__" in globals():
    __path__ = extend_path(__path__, __name__)

Platform

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Platform

• OS version and name: Linux 6.1.0-21-amd64 SMP PREEMPT_DYNAMIC Debian 6.1.90-1 (2024-05-03) x86_64 GNU/Linux
• Python version: 3.11.2
• Pelican version: HEAD (513abbf)
• Link to theme: m.css
• Links to plugins: pelican-plugins
• Link to your site: n/a
• Link to your source: n/a