Prepare for v2.0.0b1 release

- Update CHANGELOG.md
- Update to bundled capnproto-1.0.1
  * Compiles with capnproto-0.8.0 and higher
- *Breaking Change* Remove allow_cancellation (see
  https://capnproto.org/news/2023-07-28-capnproto-1.0.html)
  * This is tricky to handle for older versions of capnproto. Instead of
    dealing with lots of complication, removing it entirely.
- Fix some documentation after the build backend support was added
- Update tox.ini to support 3.8 to 3.12
- Update cibuildwheel to 2.16.1
  * Adds Python 3.12 supports and implicitly deprecates EOL 3.7 (though it's
    still built)

.github/workflows/wheels.yml

@@ -48,10 +48,10 @@ jobs:
       # Used to host cibuildwheel
       - uses: actions/setup-python@v4
         with:
-          python-version: '3.11'
+          python-version: '3.12'
 
       - name: Install cibuildwheel
-        run: python -m pip install cibuildwheel==2.12.0
+        run: python -m pip install cibuildwheel==2.16.1
 
       - name: Build wheels
         run: python -m cibuildwheel --output-dir wheelhouse