fix: setup.py can now find numpy

setup.py

@@ -1,10 +1,10 @@
 from setuptools import setup, Extension
-import numpy
+from setuptools.command.build_ext import build_ext as _build_ext
 from fastgrab import metadata
 
 module_info = Extension(
     "fastgrab._linux_x11",
-    include_dirs=[numpy.get_include()],
+    include_dirs=[],  # Populated by build_ext().
     libraries=['X11', 'gomp'],
     extra_compile_args=[
         '-fno-strict-aliasing',
@@ -15,12 +15,25 @@
     sources=["fastgrab/linux_x11/screenshot.c"]
 )
 
+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())
+
 setup(
     name=metadata.package,
     version=metadata.version,
     description=metadata.description,
     author=metadata.authors,
     url=metadata.url,
     packages=[metadata.package],
+    cmdclass={'build_ext':build_ext},
+    setup_requires=['numpy'],
     ext_modules=[module_info]
 )