GH-90: Avoid warning on python 3.11

- Upgrade ubuntu machine.

.github/workflows/main.yml

@@ -11,15 +11,13 @@ on:
 
 jobs:
   build:
-
-    runs-on: ubuntu-18.04
-
+    runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
-    - name: Set up Python 3.6
+    - uses: actions/checkout@v3
+    - name: Set up Python 3.10
       uses: actions/setup-python@v4
       with:
-        python-version: '3.6'
+        python-version: '3.10'
     - name: Install dependencies
       run: | 
         pip install coverage

mdutils/tools/TableOfContents.py

@@ -37,7 +37,7 @@ def _loop_through(self, elements, tab: str = "", depth: int = 1) -> str:
                     + "* ["
                     + item
                     + "](#"
-                    + re.sub("[^a-z0-9_\-]", "", item.lower().replace(" ", "-"))
+                    + re.sub("[^a-z0-9_-]", "", item.lower().replace(" ", "-"))
                     + ")"
                 )