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

Start by reviewing readline's existing private version names and the backend-specific behavior described in the issue. Done means public build-time and runtime version information is exposed with the requested names and fields, while the private names remain available and the libedit values describe its emulated API.