MAINT: Update versions of Github actions

Switching from ubuntu-latest to ubuntu-20.04:

* ubuntu-latest does NOT have Python 3.6 anymore
* ubuntu-20.04 has Python 3.6
* Ubuntu 20.04 is an LTS version

.github/workflows/tests.yml

@@ -4,15 +4,15 @@ on: [pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install camelot with dependencies
@@ -31,9 +31,9 @@ jobs:
         python-version: [3.9]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install camelot with dependencies