Add more tox factors for Python 3.8 and SQLAlchemy 1.4

Author: zsiciarz

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pre-commit,py{27,35,36,37}-sql{11,12,13}
+envlist = pre-commit,py{36,37,38}-sql{11,12,13,14}
 skipsdist = true
 minversion = 3.7.0
 
@@ -9,6 +9,7 @@ deps =
     sql11: sqlalchemy>=1.1,<1.2
     sql12: sqlalchemy>=1.2,<1.3
     sql13: sqlalchemy>=1.3,<1.4
+    sql14: sqlalchemy>=1.4,<1.5
 commands =
     pytest {posargs:graphene_sqlalchemy --cov=graphene_sqlalchemy}