Drop Py37, improve typing and docstrings (#544)

Drops Py3.7, improves typing and adds a mypy config, improves the docstrings and isorts the imports.

* configure isort
* sort with isort
* add github action
* enable flake8-docstrings
* fix docstrings
* add mypy env
* no implicit optional
* add type for raw
* add some type hints

src/escpos/codepages.py

@@ -1,23 +1,31 @@
+"""Helper module for codepage handling."""
 from .capabilities import CAPABILITIES
 
 
 class CodePageManager:
-    """Holds information about all the code pages (as defined
-    in escpos-printer-db).
+    """Holds information about all the code pages.
+
+    Information as defined in escpos-printer-db.
     """
 
     def __init__(self, data):
+        """Initialize codepage manager."""
         self.data = data
 
     def get_all(self):
+        """Get list of all codepages."""
         return self.data.values()
 
     @staticmethod
     def get_encoding_name(encoding):
-        # TODO resolve the encoding alias
+        """Get encoding name.
+
+        .. todo:: Resolve the encoding alias.
+        """
         return encoding.upper()
 
     def get_encoding(self, encoding):
+        """Return the encoding data."""
         return self.data[encoding]