apply flake8, remove Python 3.10 Syntax

make sure that the package works with older Python versions:

- replace matches with if-then-else
- Replace "|" with "Union"
- Remove "TypeAbbrev"

Make sure taht flake8 produces few warnings.

Add github action for automatic checks.

docs/source/conf.py

@@ -21,12 +21,12 @@ author = 'Thomas Tuerk'
 extensions = [
     'sphinx.ext.duration',
     'sphinx.ext.doctest',
-    'sphinx.ext.autodoc',    
+    'sphinx.ext.autodoc',
     'sphinx.ext.autosummary',
 ]
 
 templates_path = ['_templates']
-exclude_patterns = [] # type: ignore
+exclude_patterns = []  # type: ignore
 
 language = 'de'
 
@@ -53,4 +53,4 @@ latex_elements = {
 
 autodoc_type_aliases = {
     'SqlValue': 'SqlValue'
-}
+}