Disable frozen modules by default, add a toggle

[krassowski]

Fixes #1198

Frozen modules were extended to modules imported by cPython on startup and enabled by default with the intend of improving startup performance by up to 15% (see python/cpython#28335)

I would suggest we disable frozen modules by default because the performance difference in startup time is ~1% for ipykernel (4 to 5ms compared to total startup time of ~430ms) and this prevents user from getting difficult to diagnose errors during debugging. Of note the same 5ms makes for ~3% of plain Python startup time in 3.11.

Local micro benchmark (1000 repeats each) on Python 3.11 and 3.12.2 (using hyperfine).

Plain Python startup:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
python3.11 -Xfrozen_modules=on -c 'pass'	155.3 ± 16.8	138.5	315.1	1.00
python3.11 -Xfrozen_modules=off -c 'pass'	159.5 ± 18.8	140.9	371.2	1.03 ± 0.16
python3.12 -Xfrozen_modules=on -c 'pass'	138.2 ± 13.3	124.9	338.8	1.00
python3.12 -Xfrozen_modules=off -c 'pass'	141.6 ± 11.5	128.5	281.9	1.02 ± 0.13

hyperfine --runs 1000 "python -Xfrozen_modules=on -c 'pass'" "python -Xfrozen_modules=off -c 'pass'" --export-markdown x_frozen_on_off_plain_python_pass_1000.md

ipykernel import:

Command	Mean [ms]	Min [ms]	Max [ms]	Rel.
python3.11 -Xfrozen_modules=on -c 'import ipykernel'	428.0±33.0	382.5	648.2	1.00
python3.11 -Xfrozen_modules=off -c 'import ipykernel'	432.4±33.3	388.5	633.5	1.01 ± 0.11
python3.12 -Xfrozen_modules=on -c 'import ipykernel'	468.0±30.5	423.7	750.6	1.00
python3.12 -Xfrozen_modules=off -c 'import ipykernel'	472.4±29.3	425.8	664.5	1.01 ± 0.09

hyperfine --runs 1000 "PYDEVD_DISABLE_FILE_VALIDATION=1 python -Xfrozen_modules=on -c 'import ipykernel'" "python -Xfrozen_modules=on -c 'import ipykernel'" "python -Xfrozen_modules=off -c 'import ipykernel'" --export-markdown x_frozen_on_off_ipykernel_import_1000.md

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
PYDEVD_DISABLE_FILE_VALIDATION=1 python3.11 -Xfrozen_modules=on -c 'import ipykernel'	427.0 ± 30.9	383.4	642.7	1.00
PYDEVD_DISABLE_FILE_VALIDATION=1 python3.12 -Xfrozen_modules=on -c 'import ipykernel'	471.1 ± 36.1	425.3	751.1	1.01 ± 0.10

Users can still re-enable freezing modules by passing --frozen_modules argument to the python -m ipykernel install

Existing users will need to regenerate their kernelspec to get the benefit of this change (or add -Xfrozen_modules=off manually).

[krassowski]

I think the downstream test failure is due to a flaky test as it was passing on my fork.

[blink1073]

LGTM, thanks!