Add structured version info for decimal

Feature or enhancement

decimal.__libmpdec_version__ (from mpd_version()) is the only information about the libmpdec library, and it only describes the library loaded at runtime. The version of the library that the module was built against is not exposed, although it can differ now that only a system libmpdec is used.

libmpdec provides MPD_VERSION, MPD_MAJOR_VERSION, MPD_MINOR_VERSION and MPD_MICRO_VERSION at compile time, and mpd_version() at runtime.

Add decimal.LIBMPDEC_VERSION and decimal.LIBMPDEC_VERSION_INFO for the build-time version, decimal.libmpdec_version as an alias of __libmpdec_version__, and decimal.libmpdec_version_info for the runtime version.

These names are specific to the C implementation and are not added to _pydecimal; the test which checks that both implementations expose the same names has to exclude them.

Linked PRs

• gh-157476

首先定位 decimal 的 C 实现和 _pydecimal 实现，然后检查现有的 mpd_version() 和 libmpdec_version 处理。检查要求两个实现公开相同名称的测试；当 C 实现公开所请求的构建时和运行时名称，并且一致性测试排除这些名称时，即表示完成。