Change DEFAULT_STUB_SHEBANG to use python3 instead of python.

This reflects the change in recent Linux distributions that no longer ship a /usr/bin/python binary.

I'm not sure where this is actually used, now that if I understand correctly the shebang defaults to the Python binary found via auto-detection, but we should probably still fix this path (or remove this default fallback completely?).

Related to bazelbuild#11201. Closes bazelbuild#13851.

PiperOrigin-RevId: 392427248

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/python/PyRuntimeInfoApi.java

@@ -45,7 +45,7 @@
     category = DocCategory.PROVIDER)
 public interface PyRuntimeInfoApi<FileT extends FileApi> extends StarlarkValue {
 
-  static final String DEFAULT_STUB_SHEBANG = "#!/usr/bin/env python";
+  static final String DEFAULT_STUB_SHEBANG = "#!/usr/bin/env python3";
 
   @StarlarkMethod(
       name = "interpreter_path",