gh-140328: Use interned versions of string constants if they're already present

[albertedwardson]

• Issue: Use interned versions of string constants if they're already present #140328

[albertedwardson]

ThreeTwo questions I don't have answers for yet:

- Are increfs and decrefs for interned_dict needed?

• Why is the utf-8 string still not interned?

Python 3.15.0a0 (heads/main-dirty:5c942f11cd, Oct 27 2025, 23:16:01) [GCC 13.3.0] on linux
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>>> import sys
>>> is_interned = lambda s: s is sys.intern(('_' + s + '_')[1:-1])
>>> is_interned('utf-8')
False
>>> is_interned('str')
True

• And the most important one: am I doing this correctly?

Will try to figure it out by myself but also will be very glad if someone could guide me or even point to the answers.

[albertedwardson]

Why is the utf-8 string still not interned?

I think because it is not present in interned_dict in the first place

[albertedwardson]

will investigate why ft build fails