my.pdfs: cleanup/refactor

- modernize:
  - add REQUIRES spec for pdfannots library
  - config dataclass/config stub
  - stats function
  - absolute my.core imports in anticipation of splitting core
- use 'paths' instead of 'roots' (better reflects the semantics), use get_files
  backward compatible via config migration
- properly run tests/mypy

my/core/stats.py

@@ -29,6 +29,8 @@ def is_data_provider(fun) -> bool:
     1. returns iterable or something like that
     2. takes no arguments? (otherwise not callable by stats anyway?)
     """
+    # todo maybe for 2 allow default arguments? not sure
+    # one example which could benefit is my.pdfs
     if fun is None:
         return False
     # todo. uh.. very similar to what cachew is trying to do?