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/common.py

@@ -164,7 +164,9 @@ def get_files(
                 warnings.warn(f"{caller()}: treating {gs} as glob path. Explicit glob={glob} argument is ignored!")
             paths.extend(map(Path, do_glob(gs)))
         elif src.is_dir():
-            gp: Iterable[Path] = src.glob(glob) # todo not sure if should be recursive?
+            # todo not sure if should be recursive?
+            # note: glob='**/*.ext' works without any changes.. so perhaps it's ok as it is
+            gp: Iterable[Path] = src.glob(glob)
             paths.extend(gp)
         else:
             if not src.is_file():