Migrate from Flask to FastAPI (#163)

* Migrate from Flask to FastAPI

 * FastAPI migration:
    - Use pydantic model classes as input parameters to the
      data/calculation classes.
    - Interface field names changed to constructor parameter names (for
      simplicity only during transition, should be updated in a followup
      PR).
    - Add basic interface requirements (e.g. some values > 0, etc.).
 * Update tests for new data format.
 * Python requirement down to 3.9 (TypeGuard no longer needed)
 * Makefile: Add helpful targets (e.g. development server with reload)

* Move API doc from README to pydantic model classes (swagger)

 * Link to swagger.io with own openapi.yml.
 * Commit openapi.json and check with pytest for changes so the
   documentation is always up-to-date.

* Streamline docker

* FastAPI: Run startup action on dev server

 * Fix config for /strompreis, endpoint still broken however.

* test_openapi: Compare against docs/.../openapi.json

* Move fastapi to server/ submodule

 * See #187 for new repository structure.

src/akkudoktoreos/class_inverter.py

@@ -1,6 +1,17 @@
+from pydantic import BaseModel, Field
+
+from akkudoktoreos.class_akku import PVAkku
+
+
+class WechselrichterParameters(BaseModel):
+    max_leistung_wh: float = Field(10000, gt=0)
+
+
 class Wechselrichter:
-    def __init__(self, max_leistung_wh, akku):
-        self.max_leistung_wh = max_leistung_wh  # Maximum power that the inverter can handle
+    def __init__(self, parameters: WechselrichterParameters, akku: PVAkku):
+        self.max_leistung_wh = (
+            parameters.max_leistung_wh  # Maximum power that the inverter can handle
+        )
         self.akku = akku  # Connection to a battery object
 
     def energie_verarbeiten(self, erzeugung, verbrauch, hour):