Backport PR pandas-dev#25568: BLD: Fixed pip install with no numpy

doc/source/whatsnew/v0.24.2.rst

@@ -30,6 +30,7 @@ Fixed Regressions
 - Fixed regression in :class:`TimedeltaIndex` where ``np.sum(index)`` incorrectly returned a zero-dimensional object instead of a scalar (:issue:`25282`)
 - Fixed regression in ``IntervalDtype`` construction where passing an incorrect string with 'Interval' as a prefix could result in a ``RecursionError``. (:issue:`25338`)
 - Fixed regression in :class:`Categorical`, where constructing it from a categorical ``Series`` and an explicit ``categories=`` that differed from that in the ``Series`` created an invalid object which could trigger segfaults. (:issue:`25318`)
+- Fixed pip installing from source into an environment without NumPy (:issue:`25193`)
 
 .. _whatsnew_0242.enhancements:
 

setup.py

@@ -472,6 +472,11 @@ def maybe_cythonize(extensions, *args, **kwargs):
         # Avoid running cythonize on `python setup.py clean`
         # See https://github.com/cython/cython/issues/1495
         return extensions
+    if not cython:
+        # Avoid trying to look up numpy when installing from sdist
+        # https://github.com/pandas-dev/pandas/issues/25193
+        # TODO: See if this can be removed after pyproject.toml added.
+        return extensions
 
     numpy_incl = pkg_resources.resource_filename('numpy', 'core/include')
     # TODO: Is this really necessary here?
@@ -480,11 +485,8 @@ def maybe_cythonize(extensions, *args, **kwargs):
                 numpy_incl not in ext.include_dirs):
             ext.include_dirs.append(numpy_incl)
 
-    if cython:
-        build_ext.render_templates(_pxifiles)
-        return cythonize(extensions, *args, **kwargs)
-    else:
-        return extensions
+    build_ext.render_templates(_pxifiles)
+    return cythonize(extensions, *args, **kwargs)
 
 
 def srcpath(name=None, suffix='.pyx', subdir='src'):