utils/exporter: fix type hint for export()

(cherry picked from commit b29450c19034cf34856cf6e2a610507411833e36)

poetry/utils/exporter.py

@@ -31,7 +31,7 @@ class Exporter(object):
         dev=False,
         extras=None,
         with_credentials=False,
-    ):  # type: (str, Path, Union[IO, str], bool, bool, bool) -> None
+    ):  # type: (str, Path, Union[IO, str], bool, bool, Optional[Union[bool, Sequence[str]]], bool) -> None
         if fmt not in self.ACCEPTED_FORMATS:
             raise ValueError("Invalid export format: {}".format(fmt))