Fix utils.py import for Python 3.6

requirements.txt

@@ -1,14 +1,15 @@
-Pillow         >= 5.4.1, < 6
-pymediainfo    >= 4.1,   < 5
-cairosvg       >= 2.4.2, < 3
-aiofiles       >= 0.4.0, < 0.5
-appdirs        >= 1.4.3, < 2
-filetype       >= 1.0.5, < 2
-html_sanitizer >= 1.7.3, < 2
-lxml           >= 4.4.2, < 5
-mistune        >= 0.8.4, < 0.9
-dataclasses    >= 0.6,   < 0.7; python_version < "3.7"
-pyfastcopy     >= 1.0.3, < 2;   python_version < "3.8"
-blist          >= 1.3.6, < 2
+Pillow          >= 5.4.1, < 6
+pymediainfo     >= 4.1,   < 5
+cairosvg        >= 2.4.2, < 3
+aiofiles        >= 0.4.0, < 0.5
+appdirs         >= 1.4.3, < 2
+filetype        >= 1.0.5, < 2
+html_sanitizer  >= 1.7.3, < 2
+lxml            >= 4.4.2, < 5
+mistune         >= 0.8.4, < 0.9
+blist           >= 1.3.6, < 2
+dataclasses     >= 0.6,   < 0.7; python_version < "3.7"
+async_generator >= 1.10,  < 2;   python_version < "3.7"
+pyfastcopy      >= 1.0.3, < 2;   python_version < "3.8"
 
 git+http://github.com/mirukan/matrix-nio#egg=matrix-nio[e2e]

src/backend/utils.py

@@ -7,8 +7,8 @@ import html
 import inspect
 import io
 import json
+import sys
 import xml.etree.cElementTree as xml_etree  # FIXME: bandit warning
-from contextlib import asynccontextmanager
 from datetime import datetime, timedelta
 from enum import Enum
 from enum import auto as autostr
@@ -28,6 +28,11 @@ from aiofiles.threadpool.text import AsyncTextIOWrapper
 from nio.crypto import AsyncDataT as File
 from nio.crypto import async_generator_from_data
 
+if sys.version_info >= (3, 7):
+    from contextlib import asynccontextmanager
+else:
+    from async_generator import asynccontextmanager
+
 Size = Tuple[int, int]
 auto = autostr