MNT: Ignore differing stub for GlyphIndexType

This may be an upstream bug [1], but until that is determined, ignore
the error to get CI working.

[1] python/mypy#19877

Author: QuLogic

ci/mypy-stubtest-allowlist.txt

@@ -49,3 +49,7 @@ matplotlib\.figure\.FigureBase\.get_figure
 
 # getitem method only exists for 3.10 deprecation backcompatability
 matplotlib\.inset\.InsetIndicator\.__getitem__
+
+# Avoid a regression in NewType handling for stubtest
+# https://github.com/python/mypy/issues/19877
+matplotlib\.ft2font\.GlyphIndexType\.__init__