gh-106078: Isolate decimal module

[CharlieZhao95]

Convert decimal global state to true module state.

• Issue: Isolate the _decimal extension module #106078

[CharlieZhao95]

cc: @kumaraditya303 @erlend-aasland

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @kumaraditya303 for commit 50d5243 🤖

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[AlexWaygood]

Following this PR, I now get a warning printed to my terminal whenever I run python -m test test_decimal locally on my Windows machine inside my CPython clone:

>python -m test test_decimal
Running Release|x64 interpreter...
0:00:00 Run tests sequentially
0:00:00 [1/1] test_decimal
C:\Users\alexw\coding\cpython\Modules\_decimal\libmpdec\context.c:57: warning: mpd_setminalloc: ignoring request to set MPD_MINALLOC a second time

(I bisected the warning to this PR)

[CharlieZhao95]

Yes. The warning occures here, which is caused by the multi-phase initialization of decimal module. @AlexWaygood

This warning does not affect the module, because a static flag is used in mpd_setminalloc to ensure that MPD_MINALLOC can only be initialized once.

Maybe we can also add a static flag to the _decimal_exec to suppress this warning. But I'm not sure if it makes sense to do so (we add a static variable just to tell the compiler that we know a warning will be raised here). :)

[sunmy2019]

I think it is reasonable. Warnings are disrupting.