Updated ubuntu os to 20.04 (#431)

ubnutu 18.04 is near end of support and hence we are deprecating the support.

Author: chinmayshah99

.github/workflows/publish.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       max-parallel: 12
       matrix:
-        os: [ubuntu-18.04, macos-latest, windows-latest]
+        os: [ubuntu-20.04, macos-latest, windows-latest]
         python-version: [3.6, 3.7, 3.8, 3.9]
     runs-on: ${{ matrix.os }}