Revert fbitset bump due to ABI incompatibility with libparenth

[Copilot]

The Dependabot fbitset bump from 7d18fd7 to 3a54534 causes segmentation faults in tests due to ABI-breaking changes incompatible with libparenth.

Root Cause

fbitset 3a54534 (Oct 2019) introduced breaking changes:

• Size type: int → size_t
• Iterator internals reworked
• N_LIMBS made dynamically calculated

libparenth (last updated Sep 2019, commit 2093109) predates these changes and depends on fbitset 1f1a3db. When libparenth's template code compiles against the new fbitset headers, the ABI mismatch causes crashes in Bsums_it::incr during Python object division operations.

Changes

• Reverted deps/fbitset from 3a54534 → 7d18fd7
• Undid experimental setup.py include path changes

Path Forward

To upgrade fbitset, libparenth must first be updated for compatibility with fbitset ≥ Oct 2019.

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[Copilot]

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[coveralls]

Pull Request Test Coverage Report for Build 19003164134

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+91.2%) to 91.198%

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Totals
Change from base Build 19002992482:	91.2%
Covered Lines:	1782
Relevant Lines:	1954

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💛 - Coveralls