Previously if an import of a dependent module (usb1, hid) failed, import
of the whole transport module would fail. This was resolved by catching
ImportErrors in the all_transports method.
This had two drawbacks:
- if something other than ImportError happened - e.g., libusb would
raise OSError if it couldn't find libusb.so - all_transports would crash
anyway
- at the same time, if a legitimately needed dependency
(typing_extensions) was missing, this would be masked by the ImportError
handling.
Instead, we unconditionally import the modules, and inside each one,
wrap dependencies in a try-except.
As an added benefit, it is now possible to disable a transport just by
setting SomeTransport.ENABLED = False
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