Make python SDK targets configurable

[jsharpe]

Make the python sdk targets configurable so that they can be overriden with e.g.@python_3_12//:python_headers to avoid needing to transition the current python toolchain to another version.

Fixes:

• [Question] - How does someone build for multiple versions of python? #100

[Mizux]

Seems to follow the intended use of rules_python
src: https://github.com/bazel-contrib/rules_python/blob/main/tests/cc/current_py_cc_libs/BUILD.bazel

question: why not naming them py_cc_header and py_cc_libs i.e.

 py_cc_headers = "@rules_python//python/cc:current_py_cc_headers",

[jsharpe]

Not particularly attached to any name in particular but I named it based on the target you'd typically override it with i.e. https://github.com/bazel-contrib/rules_python/blob/5281261a97235099a65aaeb83b74b30d16409798/python/private/hermetic_runtime_repo_setup.bzl#L110 - the python_headers target from a specific python version e.g. @python_3_10//:python_headers.