Add the .py3 extension as a valid Python source.

RELNOTES: None.
PiperOrigin-RevId: 317823269
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyRuleClasses.java b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyRuleClasses.java
index 88e0b60..f91f470 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyRuleClasses.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyRuleClasses.java
@@ -45,7 +45,7 @@
  * Bazel-specific rule definitions for Python rules.
  */
 public final class BazelPyRuleClasses {
-  public static final FileType PYTHON_SOURCE = FileType.of(".py");
+  public static final FileType PYTHON_SOURCE = FileType.of(".py", ".py3");
 
   public static final LabelLateBoundDefault<?> PY_INTERPRETER =
       LabelLateBoundDefault.fromTargetConfiguration(