firmware: don't use Python 3.10 features

pybricks/support#626 (comment)

Author: dlech

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Fix syntax error on Python < 3.10 in `firmware` module.
+
 ## [1.0.0-alpha.26] - 2022-06-07
 
 ### Added

pybricksdev/firmware.py

@@ -5,7 +5,7 @@
 Utilities for working with Pybricks ``firmware.zip`` files.
 """
 
-from typing import Literal, TypedDict
+from typing import Literal, TypedDict, Union
 
 
 class FirmwareMetadataV100(
@@ -46,7 +46,7 @@ class FirmwareMetadataV110(
     """
 
 
-AnyFirmwareMetadata = FirmwareMetadataV100 | FirmwareMetadataV110
+AnyFirmwareMetadata = Union[FirmwareMetadataV100, FirmwareMetadataV110]
 """
 Type for data contained in ``firmware.metadata.json`` files of any version.
 """