Match codec and timer signature defaults to Python 3.15 - #9741

Summary

Match the displayed defaults of five native APIs to CPython 3.15.0's Argument Clinic declarations:

• binascii.hexlify and its b2a_hex alias: bytes_per_sep=1.
• signal.setitimer: interval=0.0.
• zlib.compressobj: zdict=<unrepresentable>, not None (an explicit None is rejected).
• zlib._ZlibDecompressor: the symbolic wbits=MAX_WBITS default.

These changes use the existing FromArgs signature metadata. They preserve optional sentinels, argument conversion order and ownership; no common runtime mechanism or extra argument checks are introduced. The _ZlibDecompressor runtime default remains MAX_WBITS.

The existing signature snippet checks the raw default spellings (normalizing Clinic's line wrapping), inspect's resolved defaults where representable, and omitted-versus-explicit dictionary behavior.

Reference

Compared directly with the generated Clinic headers at CPython v3.15.0:

• Modules/clinic/binascii.c.h
• Modules/clinic/signalmodule.c.h
• Modules/clinic/zlibmodule.c.h

Validation

• Clean workspace tests and strict Clippy: passed.
• C API tests and strict Clippy: passed.
• Clean release build and signature snippet: passed.
• test_binascii, test_zlib, test_signal, test_inspect: 760 tests, 93 skipped, passed.
• Existing pytest RustPython signature snippet wrapper: passed.
• The new assertions fail on the preserved pre-change RustPython binary.

CPython 3.15 execution will be checked by the existing CI snippet job; local comparisons against older CPython versions are not being used as the reference.