Update tox.ini to remove py35

tox.ini

 [tox]
-envlist = packaging, py35, py36, py37, py38, py39, check_codestyle, check_isort
+envlist = packaging, py36, py37, py38, py39, check_codestyle, check_isort
+
+# we require tox>=2.3.2 for the fix to https://github.com/tox-dev/tox/issues/208
+minversion = 2.3.2
 
 [base]
 deps =
@@ -48,6 +51,7 @@ deps =
 extras =
     # install the optional dependendencies for tox environments without
     # '-noextras' in their name
+    # (this requires tox 3)
     !noextras: all
     test
 
@@ -74,8 +78,6 @@ commands =
     # we use "env" rather than putting a value in `setenv` so that it is not
     # inherited by other tox environments.
     #
-    # keep this in sync with the copy in `testenv:py3-old`.
-    #
     /usr/bin/env COVERAGE_PROCESS_START={toxinidir}/.coveragerc "{envbindir}/trial" {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
 
 # As of twisted 16.4, trial tries to import the tests as a package (previously
@@ -121,11 +123,7 @@ commands =
     # Install Synapse itself. This won't update any libraries.
     pip install -e ".[test]"
 
-    # we have to duplicate the command from `testenv` rather than refer to it
-    # as `{[testenv]commands}`, because we run on ubuntu xenial, which has
-    # tox 2.3.1, and https://github.com/tox-dev/tox/issues/208.
-    #
-    /usr/bin/env COVERAGE_PROCESS_START={toxinidir}/.coveragerc "{envbindir}/trial" {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
+    {[testenv]commands}
 
 [testenv:benchmark]
 deps =