Add structured version info for readline

Feature or enhancement

readline exposes the version of the readline library only under private names: _READLINE_VERSION (RL_READLINE_VERSION, an int like 0x0803), _READLINE_RUNTIME_VERSION (rl_readline_version, an int) and _READLINE_LIBRARY_VERSION (rl_library_version, a string like '8.3'), added for tests.

Add readline.READLINE_VERSION_INFO for the build-time version and readline.readline_version_info for the runtime version, named tuples with major and minor fields (readline has no third component), and readline.readline_version for the runtime version string. The private names are kept for backward compatibility.

With the libedit backend (readline.backend == 'editline', i.e. the module was built against libedit's readline emulation), these describe the emulated readline API (4.2, 'EditLine wrapper'), not libedit itself, which has no meaningful version API.

Linked PRs

• gh-157486

首先检查 readline 现有的私有版本名称，以及 issue 中描述的后端特定行为。完成的标准是：使用所要求的名称和字段公开构建时和运行时的版本信息，同时保留私有名称，并确保 libedit 的值描述其模拟的 API。