Test: Modify envlist in tox.ini to check compatibility of Sanic releases

tox.ini

@@ -1,27 +1,26 @@
 [tox]
 
-envlist = py35, py36, flake8
-
-[travis]
-
-python =
-    3.5: py35, flake8
-    3.6: py36, flake8
+envlist = {py35, py36, py37}-sanic{lts, 19.03.1}, check
 
 [testenv]
 
 deps =
-    sanic
-    pytest
-    beautifulsoup4
-    aiohttp
+    saniclts: sanic==18.12.0
+    saniclts: aiohttp==3.5.4
+    sanic19.03.1: sanic==19.03.1
+    sanic19.03.1: aiohttp==3.5.4
 
 commands =
-    pytest tests {posargs}
+    pip install -e .['test']
+    pytest tests/ {posargs}
 
-[testenv:flake8]
+[testenv:check]
 deps =
-    flake8
+    black==19.3b0
+    flake8==3.7.7
+    isort==4.3.19
 
 commands =
     flake8 --max-line-length=120 sanic_openapi
+    black --check --verbose sanic_openapi
+    isort --check-only --recursive sanic_openapi