Python: Allow importing python compat flag from python source code

[ryanking13]

Related: #5678

We would like to make a small breaking change in Python, but it seems like currently importing compat flags are only available in the JS code for Python workers.

This PR enables importing compat flags from python code of Python workers such as

import _cloudflare_compat_flags
_cloudflare_compat_flags.python_workflows

This PR includes updating the custom serializer for the snapshot as the compat flag object is exposed as a global object.

[codspeed-hq]

CodSpeed Performance Report

Merging #5713 will not alter performance

_{Comparing python-compat-flag (1a46099) with main (177fb2f)}

Summary

✅ 140 untouched
⏩ 38 skipped¹

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[dom96]

Code changes look good.

One question I have is how does this interact with our baseline snapshots? We have to be careful to not serialise the compat flags in that snapshot as it could then cause issues.

[dom96]

Suggested change

	* Global objects that we need a custom serializer
	* Global objects that we need a custom serializer for

[hoodmane]

What about "Global objects that need a custom serializer"

[dom96]

bike-shedding but maybe this should be _cf_private.compat_flags just to future proof a bit, so that if we get more of these private modules, they have a nice home

[ryanking13]

One question I have is how does this interact with our baseline snapshots? We have to be careful to not serialise the compat flags in that snapshot as it could then cause issues.

What I thought was that because this uses a custom serializer, the compat flags will pick up the fresh, user-passed flags when deserializing the snapshot. @hoodmane Could you confirm that I am understanding it correctly?

[hoodmane]

That's right. Though if you say a = compat_flags.foo then obviously that is not going to be updated.

[hoodmane]

Looks good to me.