Remove an obsolete attribute from __attrs__
The extra attribute in __attrs__ makes an unpickled session have one more attribute than the original.  Removing it makes the original and the unpickled be identical.
@@ -355,7 +355,7 @@ class Session(SessionRedirectMixin):
__attrs__ = [
'headers', 'cookies', 'auth', 'proxies', 'hooks', 'params', 'verify',
- 'cert', 'prefetch', 'adapters', 'stream', 'trust_env',
+ 'cert', 'adapters', 'stream', 'trust_env',
'max_redirects',
]
