Enable mutable types for fbar TARGETS

Summary: Enable mutable thrift-python for fbar targets and their dependencies.

Reviewed By: ahilger

Differential Revision: D66619497

fbshipit-source-id: 7d5eb91103c22102834dfdd4e7a600b0f021c52f

fb303/thrift/BUCK

@@ -46,6 +46,7 @@ thrift_library(
     thrift_cpp2_options = "frozen2,reflection",
     thrift_java_swift_options = ["deprecated_allow_leagcy_reflection_client"],
     thrift_js_options = ["node"],
+    thrift_python_options = ["experimental_generate_mutable_types"],
     thrift_rust_options = [
         "deprecated_default_enum_min_i32",
         "serde",