py_cc_toolchain should contain EXT_SUFFIX

[jhance]

See https://github.com/bazelbuild/bazel-central-registry/pull/4539#discussion_r2080143661 for some context.

This value is needed to create python extensions. Note that there are a few different names that can be used, for example .so, .abi3.so, and .cpython-312-x86_64-linux-gnu.so. The latter is preferable when not using the python limited api because it prevents runfiles conflicts between python versions properly, meaning that transitions can work without completely breaking things.

I believe for autoconfigured pythons, you could obtain this value like so:

❯ python -c 'import sysconfig; print(sysconfig.get_config_var("EXT_SUFFIX"))'
.cpython-312-x86_64-linux-gnu.so