fix(setup): drop legacy numpy setup-flag check and declare runtime numpy dep

two follow-on fixes to the cherry-picked install fix from arcitec's pr #19:

- the `if "__NUMPY_SETUP__" in __builtins__` guard raises
  `TypeError: argument of type 'module' is not iterable` under modern
  setuptools where `__builtins__` is a module rather than a dict. drop it —
  modern numpy doesn't set the flag anyway, so the guard is a no-op even
  when it parses.

- add `install_requires=['numpy']` so a clean `pip install fastgrab` also
  pulls numpy at install time. without it, `setup_requires` satisfied the
  build but `import fastgrab.screenshot` failed at runtime with
  `ModuleNotFoundError: numpy`. verified end-to-end on a fresh
  `python:3.11-slim` container with no numpy preinstalled — the two-line
  api now works after a single `pip install`.

Author: mherkazandjian

setup.py

@@ -18,11 +18,6 @@
 class build_ext(_build_ext):
     def finalize_options(self):
         _build_ext.finalize_options(self)
-        # Prevent numpy from thinking it's still in its setup process.
-        # NOTE: Doesn't exist in modern numpy versions.
-        if "__NUMPY_SETUP__" in __builtins__:
-            __builtins__.__NUMPY_SETUP__ = False
-        # Add the numpy header directory to our build process.
         import numpy
         self.include_dirs.append(numpy.get_include())
 
@@ -35,5 +30,6 @@ def finalize_options(self):
     packages=[metadata.package],
     cmdclass={'build_ext':build_ext},
     setup_requires=['numpy'],
+    install_requires=['numpy'],
     ext_modules=[module_info]
 )