master.PkgutilMethod: Skip module loaders that raise ValueError

e.g. in Ansible 6, ansible-core 2.13

```
[mux  2717] 23:39:11.342416 D mitogen: PkgutilMethod(): _AnsibleCollectionLoader(path=None).get_file_name('ansible.plugins') failed: ValueError('_AnsibleCollectionLoader(path=None) cannot find files for ansible.plugins, only ansible_collections.ansible.builtin.plugins')
```

mitogen/master.py

@@ -536,7 +536,7 @@ def find(self, fullname):
 
         try:
             path = loader.get_filename(fullname)
-        except (AttributeError, ImportError):
+        except (AttributeError, ImportError, ValueError):
             # - get_filename() may throw ImportError if pkgutil.find_loader()
             #   picks a "parent" package's loader for some crap that's been
             #   stuffed in sys.modules, for example in the case of urllib3: