BLD: Add Python 3.11 builds to CI

With cibuildwheel v2.9.0, CPython 3.11 wheels are now built
(by default) and as NumPy now has CPython 3.11 wheels out as of
NumPy v1.23.2, matplotlib can build CPython 3.11 wheels (though as
of 2022-08-16 contourpy does not have 3.11 wheels yet).
   - c.f. https://github.com/pypa/cibuildwheel/releases/tag/v2.9.0

Author: matthewfeickert

.github/workflows/cibuildwheel.yml

@@ -52,6 +52,19 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: Build wheels for CPython 3.11
+        uses: pypa/cibuildwheel@v2.9.0
+        env:
+          CIBW_BUILD: "cp311-*"
+          CIBW_SKIP: "*-musllinux*"
+          CIBW_MANYLINUX_X86_64_IMAGE: manylinux2014
+          CIBW_MANYLINUX_I686_IMAGE: manylinux2014
+          CIBW_BEFORE_BUILD: >-
+            pip install certifi oldest-supported-numpy &&
+            git clean -fxd build
+          MPL_DISABLE_FH4: "yes"
+          CIBW_ARCHS: ${{ matrix.cibw_archs }}
+
       - name: Build wheels for CPython 3.10
         uses: pypa/cibuildwheel@v2.9.0
         env: