core: better support for ad-hoc configs

properly reload/unload the relevant modules so hopefully no more weird hacks should be required

relevant
- https://github.com/karlicoss/promnesia/issues/340
- https://github.com/karlicoss/HPI/issues/46

tests/test_tmp_config.py

@@ -0,0 +1,33 @@
+from pathlib import Path
+import tempfile
+
+from my.core.cfg import tmp_config
+
+import pytest
+
+
+def _init_default_config():
+    import my.config
+    class default_config:
+        count = 5
+    my.config.simple = default_config  # type: ignore[attr-defined]
+
+
+def test_tmp_config() -> None:
+    ## ugh. ideally this would be on the top level (would be a better test)
+    ## but pytest imports eveything first, executes hooks, and some reset_modules() fictures mess stuff up
+    ## later would be nice to be a bit more careful about them
+    _init_default_config()
+    from my.simple import items
+    ##
+
+    assert len(list(items())) == 5
+
+    class config:
+        class simple:
+            count = 3
+
+    with tmp_config(modules='my.simple', config=config):
+        assert len(list(items())) == 3
+
+    assert len(list(items())) == 5