Fix dependencies versions

Author: Shylpx

.github/workflows/cartoframes-ci.yml

@@ -15,7 +15,7 @@ jobs:
         include:
           - os: ubuntu-20.04
             python-version: 3.5
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             python-version: 3.6
           - os: ubuntu-latest
             python-version: 3.7

tox.ini

@@ -10,6 +10,7 @@ python =
 
 [testenv]
 deps =
+    markupsafe<=2.0.1
     flake8
     pytest
     pytest-mock
@@ -20,7 +21,7 @@ commands =
 
 [testenv:unit]
 deps =
-    markupsafe==2.0.1
+    markupsafe<=2.0.1
     flake8
     pytest
     pytest-mock