[Bug]: exec_properties with select() statements fails with "'select' value has no field or method 'items'"

[chowesasana]

What happened?

When using exec_properties with Bazel select() statements in py_test rules, the build fails with:
Error: 'select' value has no field or method 'items'

Version

• rules_py version: v1.4.0+ (all versions since the fix)
• Bazel version: Any
• OS: Any

How to reproduce

Minimal reproduction:

py_test(
    name = "my_test",
    srcs = ["test.py"],
    exec_properties = select({
        "//conditions:default": {"key": "value"},
    }),
)

Any other information?

I believe this was introduced via commit 84f17d3 (PR #529) which was first released in v1.4.0

In https://github.com/aspect-build/rules_py/blob/main/py/defs.bzl#L67, the code assumes exec_properties is always a dict:

non_test_exec_properties = {k: v for k, v in exec_properties.items() if not k.startswith("test.")}

[kolbeinn]

I think what this boils down to is that the change introduced in #529 makes exec_properties a non-configurable attribute, which it should not be according to Bazel documentation.