upgrade containers to ubuntu-20.04

as ubuntu-18.04 is no longer supported, we need to start using newer
version

Author: tomato42

.github/workflows/ci.yml

@@ -24,35 +24,35 @@ jobs:
             python-version: "3.10"
             tox-env: py310
           - name: py2.7
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             tox-env: py27
           - name: py2.7 with old gmpy
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             tox-env: py27_old_gmpy
           - name: py2.7 with old gmpy2
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             tox-env: py27_old_gmpy2
           - name: py2.7 with old six
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             tox-env: py27_old_six
           - name: py2.7 with gmpy
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             tox-env: gmpypy27
           - name: py2.7 with gmpy2
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             tox-env: gmpy2py27
           - name: py3.5
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 3.5
             tox-env: py35
           - name: py3.6
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 3.6
             tox-env: py36
           - name: py3.7
@@ -93,7 +93,7 @@ jobs:
             tox-env: pypy3
           # special configurations
           - name: py2.7 with instrumental
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             python-version: 2.7
             opt-deps: ['instrumental']
           - name: code checks