Import Protocol from typing_extensions to fix py3.7

Author: i80and

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Python 3.7 support.
+
 ## [v0.4.10] - 2020-06-17
 
 ### Changed

snooty/target_database.py

@@ -4,7 +4,8 @@
 from collections import defaultdict
 from dataclasses import dataclass, field
 from docutils.nodes import make_id
-from typing import Dict, DefaultDict, NamedTuple, Sequence, List, Protocol, Optional
+from typing import Dict, DefaultDict, NamedTuple, Sequence, List, Optional
+from typing_extensions import Protocol
 from .types import normalize_target, FileId, ProjectConfig
 from .cache import Cache
 from . import n, intersphinx, specparser