feat[flipt-client-python]: Add backwards compat support for Python 3.9 #570

Job	Run time
build (Linux-x86_64, ubuntu-latest, x86_64-unknown-linux-gnu, false)	32s
build (Linux-arm64, ubuntu-latest, aarch64-unknown-linux-gnu, true)	47s
build (Linux-x86_64-musl, ubuntu-latest, x86_64-unknown-linux-musl, true)	1m 2s
build (Linux-arm64-musl, ubuntu-latest, aarch64-unknown-linux-musl, true)	1m 13s
build (Darwin-arm64, macos-latest, aarch64-apple-darwin, false)	33s
build (Darwin-x86_64, macos-latest, x86_64-apple-darwin, false)	28s
build (Windows-x86_64, windows-latest, x86_64-pc-windows-msvc, false)	2m 4s
build (iOS-arm64, macos-latest, aarch64-apple-ios, false)	26s
build (iOS-arm64-sim, macos-latest, aarch64-apple-ios-sim, false)	33s
build (Android-arm64, ubuntu-latest, aarch64-linux-android, true)	1m 0s
build (Android-x86_64, ubuntu-latest, x86_64-linux-android, true)	59s
notify	2s
	9m 39s

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