Fix Support fo `venv`

[seanlaw]

There have been several reported issues:

#730 (comment)
#957 (comment)

where test.sh seems to exhibit strange behavior where it appears to be executing black in the home/root directory! However, what's really happening is that developers are setting up a virtual environment INSIDE of the stumpy directory:

cd stumpy.git
python -m venv .venv  # Creates a `.venv` directory that houses its own isolated Python environment
source .venv/bin/activate
./setup.sh ci  # Or `./setup.sh dev` would work as well
./test.sh

Then, black looks inside of the .venv directory and discovers a tonne of files that do not conform to black standards and that are not technically included in stumpy. flake8 also experiences a similar issue! Thus, we need to remedy this by having both black and flake8 ignore the presence of a .venv directory by adding a --extend-exclude=".venv" flag to the test.sh file