A spew of experimental errors always pops up when running dagit,
which can be a little verbose.
Although this can be supressed by adding "warnings.simplefilter" to your
user code, it sounds tedious to put that all over the place.
Instead, add a flag to suprress all warnings in the user code. Unfortunately,
using a context manager is not enough since repository locations are loaded
in individual threads, and warning supression is not thread safe.
To get around this, we set the PYTHONWARNINGS environment variable.