FIX: MouseButton representation in boilerplate generated signatures

In Python 3.10 the repr and str representation of Enums changed from

 str: 'ClassName.NAME' -> 'NAME'
 repr: '<ClassName.NAME: value>' -> 'ClassName.NAME'

which is more consistent with what str/repr should do, however this breaks
boilerplate which needs to get the ClassName.NAME version in all versions of
Python. Thus, we locally monkey patch our preferred str representation in
here.

bpo-40066
python/cpython#22392

tools/boilerplate.py

@@ -24,9 +24,31 @@
 import numpy as np
 from matplotlib import _api, mlab
 from matplotlib.axes import Axes
+from matplotlib.backend_bases import MouseButton
 from matplotlib.figure import Figure
 
 
+# we need to define a custom str because py310 change
+# In Python 3.10 the repr and str representation of Enums changed from
+#
+#  str: 'ClassName.NAME' -> 'NAME'
+#  repr: '<ClassName.NAME: value>' -> 'ClassName.NAME'
+#
+# which is more consistent with what str/repr should do, however this breaks
+# boilerplate which needs to get the ClassName.NAME version in all versions of
+# Python. Thus, we locally monkey patch our preferred str representation in
+# here.
+#
+# bpo-40066
+# https://github.com/python/cpython/pull/22392/
+def enum_str_back_compat_patch(self):
+    return f'{type(self).__name__}.{self.name}'
+
+# only monkey patch if we have to.
+if str(MouseButton.LEFT) != 'MouseButton.Left':
+    MouseButton.__str__ = enum_str_back_compat_patch
+
+
 # This is the magic line that must exist in pyplot, after which the boilerplate
 # content will be appended.
 PYPLOT_MAGIC_HEADER = (