use ubuntu-18.04 CI image for better compatibility

.github/workflows/python-package.yml

@@ -12,7 +12,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: false
       matrix:

.github/workflows/python-publish.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-18.04, macos-latest, windows-latest]
         python-version: [3.6, 3.7, 3.8, 3.9]
         
     steps:
@@ -48,7 +48,7 @@ jobs:
       run: python setup.py bdist_wheel
         
     - name: Rename Linux wheels to supported platform of PyPI
-      if: matrix.os == 'ubuntu-latest'
+      if: matrix.os == 'ubuntu-18.04'
       run: for f in dist/*.whl; do mv "$f" "$(echo "$f" | sed s/linux/manylinux1/)"; done
 
     - name: Publish wheels to GitHub artifacts
@@ -60,7 +60,7 @@ jobs:
 
   publish-pypi:
     needs: [build-wheels]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2