Python 3.12 compatibility

[mjpieters]

Add 3.12 to the CI test matrix and ensure things work.

• Ignore depr warnings inside pytest on 3.12
• Upgrade cython to 3.0.0b1

Fixes #829

[codecov]

Codecov Report

Merging #881 (90f6431) into master (1d03a63) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #881   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files           4        4           
  Lines         772      772           
  Branches      219      219           
=======================================
  Hits          770      770           
  Misses          2        2           

Flag	Coverage Δ
unit	99.61% <ø> (ø)

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[mjpieters]

Note that the 3.12 test suite now fails because 3.12.0b1 includes an updated urllib.parse version which performs IP address validation; the same changes will be part of Python 3.11.4 (due to be released very very soon). This is a separate issue, see #876, which should be tackled in a separate PR.