feat: support --fake for aerich upgrade (#398)

* feat: support `--fake` for aerich upgrade

* Add `--fake` to downgrade

* tests: check --fake result for aerich upgrade and downgrade

* Update readme

* Fix unittest failed because of `db_field_types` changed

* refactor: improve type hints and document

tests/_utils.py

@@ -0,0 +1,46 @@
+import contextlib
+import os
+import shutil
+import sys
+from pathlib import Path
+
+from tortoise import Tortoise, generate_schema_for_client
+from tortoise.exceptions import DBConnectionError, OperationalError
+
+if sys.version_info >= (3, 11):
+    from contextlib import chdir
+else:
+
+    class chdir(contextlib.AbstractContextManager):  # Copied from source code of Python3.13
+        """Non thread-safe context manager to change the current working directory."""
+
+        def __init__(self, path):
+            self.path = path
+            self._old_cwd = []
+
+        def __enter__(self):
+            self._old_cwd.append(os.getcwd())
+            os.chdir(self.path)
+
+        def __exit__(self, *excinfo):
+            os.chdir(self._old_cwd.pop())
+
+
+async def drop_db(tortoise_orm) -> None:
+    # Placing init outside the try-block(suppress) since it doesn't
+    # establish connections to the DB eagerly.
+    await Tortoise.init(config=tortoise_orm)
+    with contextlib.suppress(DBConnectionError, OperationalError):
+        await Tortoise._drop_databases()
+
+
+async def init_db(tortoise_orm, generate_schemas=True) -> None:
+    await drop_db(tortoise_orm)
+    await Tortoise.init(config=tortoise_orm, _create_db=True)
+    if generate_schemas:
+        await generate_schema_for_client(Tortoise.get_connection("default"), safe=True)
+
+
+def copy_files(*src_files: Path, target_dir: Path) -> None:
+    for src in src_files:
+        shutil.copy(src, target_dir)