Run unit tests against python 3.7 (#4677)

* Run unit tests against python 3.7

... so that we span the full range of our supported python versions

* Switch to xenial

* fix psql fail

* pep8 etc want python 3.6

tox.ini

@@ -3,7 +3,6 @@ envlist = packaging, py27, py36, pep8, check_isort
 
 [base]
 deps =
-    Twisted>=17.1
     mock
     python-subunit
     junitxml
@@ -38,6 +37,7 @@ whitelist_externals =
 
 setenv =
     {[base]setenv}
+    postgres: SYNAPSE_POSTGRES = 1
 
 passenv = *
 
@@ -47,8 +47,6 @@ commands =
     sh -c 'echo "import coverage; coverage.process_startup()" > {envsitepackagesdir}/../sitecustomize.py'
     {envbindir}/coverage run "{envbindir}/trial" {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
 
-[testenv:py27]
-
 # As of twisted 16.4, trial tries to import the tests as a package (previously
 # it loaded the files explicitly), which means they need to be on the
 # pythonpath. Our sdist doesn't include the 'tests' package, so normally it
@@ -72,14 +70,7 @@ commands =
 # )
 usedevelop=true
 
-[testenv:py27-postgres]
-usedevelop=true
-deps =
-    {[base]deps}
-    psycopg2
-setenv =
-    {[base]setenv}
-    SYNAPSE_POSTGRES = 1
+
 
 # A test suite for the oldest supported versions of Python libraries, to catch
 # any uses of APIs not available in them.
@@ -101,22 +92,6 @@ commands =
     pip install -e .
     {envbindir}/trial {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
 
-[testenv:py35]
-usedevelop=true
-
-[testenv:py36]
-usedevelop=true
-
-[testenv:py36-postgres]
-usedevelop=true
-deps =
-    {[base]deps}
-    psycopg2
-setenv =
-    {[base]setenv}
-    SYNAPSE_POSTGRES = 1
-
-
 [testenv:packaging]
 skip_install=True
 deps =