Add user-agent to DummyConfig

Fixes this error while trying to fetch lists:

```
AttributeError: ('intersphinx inventory %r not fetchable due to %s: %s', 'http://docs.python.org/3.8/objects.inv', <class 'AttributeError'>, "'DummyConfig' object has no attribute 'user_agent'")

```

Author: sanjayankur31

stdlib_list/fetch.py

@@ -11,6 +11,7 @@ def __init__(self, intersphinx_mapping=None, intersphinx_cache_limit=5, intersph
         self.intersphinx_mapping = intersphinx_mapping or {}
         self.intersphinx_cache_limit = intersphinx_cache_limit
         self.intersphinx_timeout = intersphinx_timeout
+        self.user_agent = "python-stdlib-list"
         self.tls_verify = True