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.

tests/test_openapi.py

@@ -0,0 +1,19 @@
+import json
+from pathlib import Path
+
+from generate_openapi import generate_openapi
+
+DIR_PROJECT_ROOT = Path(__file__).parent.parent
+DIR_TESTDATA = Path(__file__).parent / "testdata"
+
+
+def test_openapi_spec_current():
+    """Verify the openapi spec hasn´t changed."""
+    old_spec_path = DIR_PROJECT_ROOT / "docs" / "akkudoktoreos" / "openapi.json"
+    new_spec_path = DIR_TESTDATA / "openapi-new.json"
+    generate_openapi(new_spec_path)
+    with open(new_spec_path) as f_new:
+        new_spec = json.load(f_new)
+    with open(old_spec_path) as f_old:
+        old_spec = json.load(f_old)
+    assert new_spec == old_spec