bpo-40334: Use old compiler when compile mode is func_type (GH-19692)

bpo-40334: Use old compiler when compile mode is func_type (GH-19692)

This is invoked by mypy, using ast.parse(source, "<func_type>", "func_type"). Since the new grammar doesn't yet support the func_type_input start symbol we must use the old compiler in this case to prevent a crash.

https://bugs.python.org/issue40334