remove optional mpl import

Author: MarcoGorelli

pymc3/plots/posteriorplot.py

@@ -18,14 +18,11 @@
 
 from pymc3.backends.base import MultiTrace
 
+import matplotlib.pyplot as plt
+
 if TYPE_CHECKING:  # pragma: nocover
     from arviz.data.inference_data import InferenceData
 
-try:
-    import matplotlib.pyplot as plt
-except ImportError:  # pragma: nocover (mpl is optional)
-    pass
-
 
 def plot_posterior_predictive_glm(
     trace: Union["InferenceData", MultiTrace],