matplotlib/matplotlib: REL: v3.3.4

This is the fourth bugfix release of the 3.3.x series. This release contains several critical bug-fixes: Fix WebAgg initialization. Fix parsing QT_API setting with mixed case. Fix build with link-time optimization disabled in environment. Fix test compatibility with Python 3.10. Fix test compatibility with NumPy 1.20. Fix test compatibility with pytest 6.2.