tests: add timeout to pytests

[ap--]

Some pytests might get stuck on reading data from the spectrometer.
This is especially a problem for pyseabreeze tests that might have an incorrect implementation for some spectrometers.

TODO: https://bitbucket.org/pytest-dev/pytest-timeout/src/default/

This is related to #69

[ap--]

I tried timeouts and it's very complicated to get this right together with the hardware backed tests.
I'll consider patching the timeout value for reading data from the spectrometer specifically for tests in the future.

(The issue is, that some tests could stall the test suite due to having to wait for the usb timeout, which is long)