Revert gRPC Python Kokoro cfg (#26678)

tools/internal_ci/linux/pull_request/grpc_basictests_python.cfg

@@ -14,16 +14,17 @@
 
 # Config file for the internal CI (in protobuf text format)
 
-# [DO-NOT-SUBMIT] hijacking the GitHub Check to run xds tests.
-build_file: "grpc/tools/internal_ci/linux/grpc_xds.sh"
-timeout_mins: 360
-env_vars {
-  key: "BAZEL_SCRIPT"
-  value: "tools/internal_ci/linux/grpc_xds_v3_bazel_test_in_docker.sh"
-}
+# Location of the continuous shell script in repository.
+build_file: "grpc/tools/internal_ci/linux/grpc_run_tests_matrix.sh"
+timeout_mins: 60
 action {
   define_artifacts {
     regex: "**/*sponge_log.*"
     regex: "github/grpc/reports/**"
   }
 }
+
+env_vars {
+  key: "RUN_TESTS_FLAGS"
+  value: "-f basictests linux python --inner_jobs 16 -j 2 --internal_ci --max_time=3600"
+}