Fix Fedora/RedHat installation location

Those OSes have separate platlib and purelib directories.
Symengine was installed partially into both from version 0.10.0 onwards.

This change marks some of it's packages as ext_modules, which makes
setuptools treat the wheel as platform specific and install if fully
into platlib (lib64) instead of purelib (lib).

This should solve #474.

Author: aostrowski-hbn

cmake/FindPython.cmake

@@ -60,7 +60,7 @@ if (${CMAKE_SYSTEM_NAME} STREQUAL "Windows")
 endif()
 
 execute_process(
-    COMMAND ${PYTHON_BIN} -c "from sysconfig import get_paths; print(get_paths()['purelib'])"
+    COMMAND ${PYTHON_BIN} -c "from sysconfig import get_paths; print(get_paths()['platlib'])"
     OUTPUT_VARIABLE PYTHON_INSTALL_PATH_tmp
 )
 string(STRIP ${PYTHON_INSTALL_PATH_tmp} PYTHON_INSTALL_PATH_tmp)

setup.py

@@ -24,7 +24,7 @@
 
 if use_setuptools:
     try:
-        from setuptools import setup
+        from setuptools import Extension, setup
         from setuptools.command.install import install as _install
         from setuptools.command.build_ext import build_ext as _build_ext
     except ImportError:
@@ -232,7 +232,8 @@ def finalize_options(self):
       url="https://github.com/symengine/symengine.py",
       python_requires='>=3.9,<4',
       zip_safe=False,
-      packages=['symengine', 'symengine.lib', 'symengine.tests'],
+      ext_modules=[Extension(name='symengine.lib', sources=[])],
+      packages=['symengine', 'symengine.tests'],
       cmdclass = cmdclass,
       classifiers=[
         'License :: OSI Approved :: MIT License',