Avoid pip version warnings on CI

Annotations
1 error
tests_py3x (3.5)
You are using pip version 20.3.4, however version 23.2.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

.github/workflows/main.yml

@@ -2,6 +2,9 @@ name: Run all tests
 
 on: [push, pull_request]
 
+env:
+  PIP_DISABLE_PIP_VERSION_CHECK: 1
+
 jobs:
   tests_py27:
     runs-on: ubuntu-20.04