Fix typo preventing startup

[kw217]

Description:

Fixes a typo in ble_gateway causing a fatal error on startup.

Checklist:

• [ x] The pull request is done against the latest development branch
• [ x] Only one feature/fix was added per PR and the code change compiles without warnings
• [ x] I accept the DCO.

[koenvervloesem]

This shouldn't fail, but there have been some changes in Python's typing in recent versions, so this is probably version-related. Can you:

• show the exact output on startup, including the fatal error and complete stack trace?
• show the output of python -m TheengsGateway.diagnose?

[koenvervloesem]

I investigated this, the current code fails on Python 3.8. Thanks for the fix!