Setting `matplotlib` backend breaks non-`tkagg` plotting

[bwohlberg]

Importing mbirjax breaks plotting in any environment that uses a backend (e.g. qtagg) other than tkagg:

mbirjax/mbirjax/viewer.py

Lines 7 to 15 in 25950c1

	# Set backend
	if os.environ.get("READTHEDOCS") == "True":
	matplotlib.use('Agg')
	else:
	try:
	matplotlib.use('TkAgg')
	except ImportError:
	matplotlib.use('Agg') # Fallback
	warnings.warn("TkAgg not available. Falling back to Agg.")

I suggest removing these and adding the READTHEDOCS test to the sphinx conf.py.