maint: do not use old versions of pip

Author: oesteban

.github/workflows/pythonpackage.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         python-version: [3.7, 3.8]
-        pip: ["pip~=18.1", "pip>=20.3"]
+        pip: ["pip~=20.3", "pip~=21.0"]
 
     steps:
     - name: Install hdf5 libraries