Fix #1949: zipapp virtual environment creation fails if zipapp path is symlinked

[HandSonic]

Original issue: #1949

Added test for pull #2613

The function in question assumes that ROOT is a prefix of full_path, and tries to remove it. However, it is not necessarily a prefix:

• when ROOT is determined, both os.path.abspath() and os.path.realpath() gets called
• when full_path is determined, only os.path.abspath() is called

This means that if the path contains a symlink, it will show up as resolved in ROOT, but not full_path, so simply removing the first X characters of full_path is not sufficient.

This PR adds an assertion to catch the bad situation early, then adds the os.path.realpath call to full_path.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation