💚 Update static-analysis Python version to 3.9

Manually disabling spurious error message arising from a bug in pylint
for Python 3.9, i.e.,

```
pylint...............................................................................Failed
- hook id: pylint
- exit code: 2

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

************* Module structlog_sentry_logger._config
structlog_sentry_logger/_config.py:240:18: E1136: Value 'Union' is unsubscriptable (unsubscriptable-object)

------------------------------------------------------------------
Your code has been rated at 9.52/10 (previous run: 9.52/10, +0.00)

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)
```
see: [[Python 3.9] Value 'Optional' is unsubscriptable
(unsubscriptable-object) (also Union) #3882](pylint-dev/pylint#3882)

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: "actions/checkout@v2"
       - uses: "actions/setup-python@v2"
         with:
-          python-version: "3.8"
+          python-version: "3.9"
       - name: "Install package"
         run: |
           pip install --constraint=.github/workflows/constraints.txt poetry

.gitlab-ci.yml

@@ -50,7 +50,7 @@ stages:
 
 static-analysis:
   <<: *quality
-  image: python:3.8
+  image: python:3.9
 
 dependency-security-vulnerability-analysis:
   <<: *install-dev-deps

structlog_sentry_logger/_config.py

@@ -237,7 +237,9 @@ def __init__(  # pylint: disable=too-many-arguments
         breadcrumb_level: int = logging.INFO,
         active: bool = True,
         as_extra: bool = True,
-        tag_keys: Union[List[str], str] = None,
+        tag_keys: Union[  # pylint: disable=unsubscriptable-object
+            List[str], str
+        ] = None,
     ) -> None:
         self.breadcrumb_level = breadcrumb_level
         super().__init__(