Fixed travis yml config warnings

.travis.yml

@@ -1,11 +1,8 @@
-os:
+os: linux
-  - linux
+dist: focal
-dist:
-  - focal
 services:
   - xvfb
-language:
+language: python
-  - python
 cache:
   - bundler
 
@@ -34,10 +31,8 @@ stages:
 
 jobs:
   include:
-    - stage:
+    - stage: Main
-      - Main
+      python: 3.8
-      python:
-      - 3.8
       script:
       - python -m pytest --cov=nw -v
       after_success:
@@ -45,15 +40,12 @@ jobs:
       after_failure:
       - cat /sys/fs/cgroup/memory/memory.max_usage_in_bytes
 
-    - stage:
+    - stage: Supported
-      - Supported
+      python: 3.6
-      python:
-      - 3.6
       script:
       - python -m pytest -v
 
-    - python:
+    - python: 3.7
-      - 3.7
       script:
       - python -m pytest -v