Use sys.monitoring for small speedup of tests in Python 3.12+ (#1220)

This adds a pytest flag to `pyproject.toml` to use sys.monitoring if it
is available. This can speed up `coverage.py`. The reason is that
`sys.monitoring` (introduced in Python 3.12 via PEP 669) offers a more
efficient way to monitor program execution, potentially leading to
faster coverage measurement, especially for line coverage. This feature
is only available for Python 3.12+, but it can be turned on always and
warnings about _not_ having the sysmon feature can be ignored.

The same change was applied in Cirq recently and it has been beneficial
there.

Author: mhucka

pyproject.toml

@@ -17,3 +17,8 @@ line-length = 100
 target_version = ['py310', 'py311', 'py312', 'py313']
 skip-string-normalization = true
 skip-magic-trailing-comma = true
+
+[tool.coverage.run]
+# Speed up pytest-cov on Python 3.12+ by enabling sys.monitoring if possible.
+core = "sysmon"
+disable_warnings = ["no-sysmon"]