Pin ubuntu to 18.04 for publishing jobs

Author: rmk135

.github/workflows/publishing.yml

@@ -9,7 +9,7 @@ jobs:
 
   tests:
     name: Run tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
@@ -22,7 +22,7 @@ jobs:
 
   linters:
     name: Run linters
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         toxenv: [flake8, pydocstyle, mypy, pylint]
@@ -39,7 +39,7 @@ jobs:
   build-sdist:
     name: Build source tarball
     needs: [tests, linters]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
@@ -56,7 +56,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest, macos-latest]
+        os: [ubuntu-18.04, windows-latest, macos-latest]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
@@ -75,7 +75,7 @@ jobs:
   build-wheels-linux-aarch64:
     name: Build wheels (ubuntu-latest-aarch64)
     needs: [tests, linters]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - name: Set up QEMU
@@ -92,7 +92,7 @@ jobs:
   publish:
     name: Publish on PyPI
     needs: [build-sdist, build-wheels, build-wheels-linux-aarch64]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/download-artifact@v2
         with:
@@ -106,7 +106,7 @@ jobs:
   publish-docs:
     name: Publish docs
     needs: [publish]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2