general: initial flake8 checks (for now manual)

fix fairly uncontroversial stuff in my.core like
- line spacing, which isn't too annoying (e.g. unlike many inline whitespace checks that break vertical formatting)
- unused imports/variables
- too broad except

my/core/cachew.py

@@ -6,7 +6,7 @@ from typing import Optional
 
 def disable_cachew() -> None:
     try:
-        import cachew
+        import cachew  # noqa: F401 # unused, it's fine
     except ImportError:
         # nothing to disable
         return
@@ -19,7 +19,7 @@ from typing import Iterator
 @contextmanager
 def disabled_cachew() -> Iterator[None]:
     try:
-        import cachew
+        import cachew  # noqa: F401 # unused, it's fine
     except ImportError:
         # nothing to disable
         yield