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/config.py

@@ -83,3 +83,7 @@ class commits:
     emails: Optional[Sequence[str]]
     names: Optional[Sequence[str]]
     roots: Sequence[PathIsh]
+
+
+class pdfs:
+    paths: Paths