Add [coverage] combine option

[icemac]

Measure coverage across all supported Python versions instead of a single one.

Doing this today costs a package a hand-written GitHub Actions expression, a depends = py310,… list maintained by hand, and raw relative_files config. [coverage] combine = true wires up all of it; anything the package configures itself still wins. Defaults are unchanged — regenerating an untouched pure-python package produces a byte-identical result.

Not supported for c-code, which has its own tests.yml.

— Comment created by Claude

These are the changes needed for zopefoundation/RestrictedPython#238

[coveralls]

Coverage Report for CI Build 31674103560

Coverage increased (+3.9%) to 18.695%

Details

• Coverage increased (+3.9%) from the base build.
• Patch coverage: 12 uncovered changes across 1 file (78 of 90 lines covered, 86.67%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
src/zope/meta/config_package.py	35	23	65.71%
Total (2 files)	90	78	86.67%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
src/zope/meta/config_package.py	1	23.53%

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Coverage Stats

Relevant Lines:	1354
Covered Lines:	326
Line Coverage:	24.08%
Relevant Branches:	470
Covered Branches:	15
Branch Coverage:	3.19%
Branches in Coverage %:	Yes
Coverage Strength:	0.24 hits per line

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💛 - Coveralls

[icemac]

Thank you for reviewing this PR. 😃