test: use non-interactive plotting backend (#138)

The use of a non-interactive plotting backend prevents plots from being
displayed in pop-up windows while running tests. The tests would
previously block when a plot was displayed until the user closed it.

Resolves #31

tests/__init__.py

@@ -0,0 +1,8 @@
+import warnings
+
+import matplotlib.pyplot as plt
+
+# switch matplotlib backend to non-Gui, preventing plots being displayed
+plt.switch_backend("Agg")
+# suppress UserWarning that the current backend cannot show plots
+warnings.filterwarnings("ignore", "Matplotlib is currently using agg")