BLD: require cython if tempita is needed

closes #14204

Author: jreback

setup.py

@@ -125,6 +125,9 @@ def is_platform_mac():
 class build_ext(_build_ext):
     def build_extensions(self):
 
+        if not cython:
+            raise ImportError('Building pandas requires cython')
+
         for pxifile in _pxifiles:
             # build pxifiles first, template extention must be .pxi.in
             assert pxifile.endswith('.pxi.in')