Fix build to avoid duplicate files in wheel

Fix the build system to package pure Python files
via distutils/setuptools, and limit CMake to installing the compiled
extension.

The prior logic has installed some of the .py files both via setuptools
and via CMake, to different build directories.  As a result,
the resulting wheel contained duplicate files, e.g.:

         2170  05-02-2022 07:08   symengine/__init__.py
         2170  05-02-2022 07:08   symengine-0.9.2.data/purelib/symengine/__init__.py

Duplicate files cause the wheel to be rejected by the installer package.

After the change, a correct wheel is generated.  Installation works
both via PEP517/wheel and via legacy `setup.py install`.

Author: mgorny

setup.py

@@ -226,7 +226,7 @@ def finalize_options(self):
       url="https://github.com/symengine/symengine.py",
       python_requires='>=3.7,<4',
       zip_safe=False,
-      packages=['symengine'],
+      packages=['symengine', 'symengine.lib', 'symengine.tests'],
       cmdclass = cmdclass,
       classifiers=[
         'License :: OSI Approved :: MIT License',

symengine/CMakeLists.txt

@@ -1,7 +1 @@
 add_subdirectory(lib)
-add_subdirectory(tests)
-
-set(PY_PATH ${PYTHON_INSTALL_PATH}/symengine)
-install(FILES __init__.py utilities.py sympy_compat.py functions.py printing.py
-    DESTINATION ${PY_PATH}
-    )

symengine/lib/CMakeLists.txt

@@ -28,7 +28,7 @@ install(TARGETS symengine_wrapper
             ARCHIVE DESTINATION ${PY_PATH}
             LIBRARY DESTINATION ${PY_PATH}
         )
-install(FILES __init__.py
+install(FILES
     ${CMAKE_CURRENT_BINARY_DIR}/config.pxi
     symengine.pxd
     symengine_wrapper.pxd