fix(lib): allow DocTest to discover methods from subclasses of eqx.Module

Closes #1016.

In the end, there were two issues: (1) `DocTestFinder._from_module` was returning `False` because the `__module__` attribute was not correct, and (2) the documentation was not copied to the wrapper class so doctests could not be discovered.

My solution is inspired from what Python is doing with their own decorators, see https://github.com/python/cpython/blob/4617d68d73409e83d6ab31106d10421d44048787/Lib/functools.py#L1033-L1049

Author: jeertmans

equinox/_module.py

@@ -680,6 +680,15 @@ def __init__(self, method):
         self.method = method
         if getattr(self.method, "__isabstractmethod__", False):
             self.__isabstractmethod__ = self.method.__isabstractmethod__
+        # Fixes https://github.com/patrick-kidger/equinox/issues/1016
+        try:
+            self.__module__ = self.method.__module__
+        except AttributeError:
+            pass
+        try:
+            self.__doc__ = self.method.__doc__
+        except AttributeError:
+            pass
 
     def __get__(self, instance, owner):
         del owner