Do not install setuptools 50.0 on Python 3.5

INSTALL.md

@@ -73,7 +73,7 @@ mkdir -p ~/synapse
 virtualenv -p python3 ~/synapse/env
 source ~/synapse/env/bin/activate
 pip install --upgrade pip
-pip install --upgrade setuptools
+pip install --upgrade setuptools!=50.0  # setuptools==50.0 fails on some older Python versions
 pip install matrix-synapse
 ```
 

changelog.d/8212.bugfix

@@ -0,0 +1 @@
+Do not install setuptools 50.0 on Python 3.5, which leads to a broken configuration.

synapse/python_dependencies.py

@@ -74,6 +74,10 @@
     "Jinja2>=2.9",
     "bleach>=1.4.3",
     "typing-extensions>=3.7.4",
+    # setuptools is required by a variety of dependencies, unfortunately version
+    # 50.0 is incompatible with older Python versions, see
+    # https://github.com/pypa/setuptools/issues/2352
+    'setuptools!=50.0',
 ]
 
 CONDITIONAL_REQUIREMENTS = {