Flip default Python version to PY3

This flips --incompatible_py3_is_default and --incompatible_py2_outputs_are_suffixed. We opt out of this change for our own analysis and shell integration tests to avoid migrating them at the moment.

Fixes #7359, fixes #7593, fixes #6647.

RELNOTES[INC]: Python 3 is now the default Python version (for `py_binary` and `py_test` targets that don't specify the `python_version` attribute). Targets that are built for Python 3 will no longer have their output put in a separate `-py3` directory; instead there is now a separate `-py2` directory for Python 2 targets. See #7359 and #7593 for more information.

PiperOrigin-RevId: 241142275
diff --git a/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java b/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
index 13ca9c9..93ba5d7 100644
--- a/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
+++ b/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
@@ -118,7 +118,10 @@
    * some reason.
    */
   public static final ImmutableList<String> PRODUCT_SPECIFIC_FLAGS =
-      ImmutableList.of();
+      ImmutableList.of(
+          // TODO(#7903): Remove once our own tests are migrated.
+          "--incompatible_py3_is_default=false",
+          "--incompatible_py2_outputs_are_suffixed=false");
 
   public static final BuilderFactoryForTesting PACKAGE_FACTORY_BUILDER_FACTORY_FOR_TESTING =
       PackageFactoryBuilderFactoryForBazelUnitTests.INSTANCE;