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_headers 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.

[Mizux]

After second thought, I'm not sure this change will:

• Interact nicely with the --@rules_python//python/config_settings:python_version=3.X ?
• need that the build of the :pybind11 target also need to use the same custom python headers
  ref:

  pybind11_bazel/pybind11-BUILD.bazel

  Line 25 in 189775d

  deps = ["@rules_python//python/cc:current_py_cc_headers"],

• Work as expected if a super build project want to integrate a repo which is hacking the python_headers/python_library in its pybind_extension() internal usage i.e. leading to mismatch and strange errors...

ps: concerning naming, I would instead use python_headers/python_library removing the _target suffix since bazel args never use this naming convention afaik...