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/serialize.py

@@ -151,8 +151,6 @@ def dumps(
 def test_serialize_fallback() -> None:
     import json as jsn  # dont cause possible conflicts with module code
 
-    import pytest
-
     # cant use a namedtuple here, since the default json.dump serializer
     # serializes namedtuples as tuples, which become arrays
     # just test with an array of mixed objects
@@ -168,7 +166,6 @@ def test_serialize_fallback() -> None:
         assert res == [5, 5.0]
 
 
-
 # this needs to be defined here to prevent a mypy bug
 # see https://github.com/python/mypy/issues/7281
 class _A(NamedTuple):