Sympy doesn't work on Python 3.6 for some reason

.gitlab-ci.yml

@@ -13,10 +13,6 @@ cache:
   script:
     - python3 -m pytest
 
-test:PY3.6:
-  <<: *test_template
-  image: python:3.6
-
 test:PY3.7:
   <<: *test_template
   image: python:3.7
@@ -25,6 +21,10 @@ test:PY3.8:
   <<: *test_template
   image: python:3.8
 
+test:PY3.9:
+  <<: *test_template
+  image: python:3.9
+
 build-package:
   stage: deploy
   image: python:3.7