Other answers did not work for me, but I found another solution.

I installed Xcode, and then the upgrade succeeded.

Additional info:

Environment: MacBook Pro 2016, macOS Monterey

Using `brew upgrade glib -v`, I found that `/usr/bin/python3` was used during the `gobject-introspection` build.

Things I tried (but failed):

- Changed `PATH` to prioritize `/usr/local/bin/python3`

- Updated `PKG_CONFIG_PATH` to include `/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/pkgconfig`

Checked `python-3.9.pc`, which had 'prefix=/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9' so,
- Changed it to 'prefix=/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9'

Since other formulae required Xcode, I installed Xcode, and that resolved the issue.

One more thing I haven't tried yet:

/usr/bin/python3 -m pip install pkgconfig

might also help.