refactor: require UseCython (like UseSWIG)

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

README.md

@@ -19,6 +19,7 @@ This provides helpers for using Cython. Use:
 
 ```cmake
 find_package(Cython MODULE REQUIRED VERSION 3.0)
+include(UseCython)
 ```
 
 If you find Python beforehand, the search will take this into account. You can

src/cython_cmake/cmake/FindCython.cmake

@@ -18,7 +18,7 @@
 #  ``Cython::Cython``
 #    The Cython executable
 #
-# A range of versions is supported on CMake 3.19+.
+# A range of versions is supported on CMake 3.19+. See also UseCython.
 #
 # For more information on the Cython project, see https://cython.org/.
 #
@@ -106,8 +106,6 @@ if(CYTHON_FOUND)
         IMPORTED_LOCATION "${CYTHON_EXECUTABLE}"
     )
   endif()
-
-  include(UseCython)
 endif()
 
 mark_as_advanced(CYTHON_EXECUTABLE)

tests/packages/simple/CMakeLists.txt

@@ -6,6 +6,7 @@ find_package(
   COMPONENTS Interpreter Development.Module
   REQUIRED)
 find_package(Cython MODULE REQUIRED VERSION 3.0)
+include(UseCython)
 
 cython_compile_pyx(simple.pyx LANGUAGE C OUTPUT_VARIABLE simple_c)