refactor: mv py_binary.bzl to py_venv_exec.bzl

[jbedard]

Changes are visible to end-users: no

Test plan

• Covered by existing test cases

[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

136 tests (100.0%) were fully cached saving 58s.

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Bazel 9 (Test)

All tests were cache hits

135 tests (100.0%) were fully cached saving 1m 6s.

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Bazel 8 (Test)

e2e

All tests were cache hits

84 tests (100.0%) were fully cached saving 49s.

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Bazel 9 (Test)

e2e

All tests were cache hits

84 tests (100.0%) were fully cached saving 51s.

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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Buildifier      Gazelle