Releasing symengine.py

1. Update symengine C++ version in https://github.com/symengine/symengine.py/blob/master/symengine_version.txt

2. Update symengine C++ version in https://github.com/symengine/symengine.py/blob/master/CMakeLists.txt#L5

3. Update symengine.py version in https://github.com/symengine/symengine.py/blob/master/setup.py#L226

4. Update symengine.py version in https://github.com/symengine/symengine-wheels and make sure tests pass there

5. Tag a release in symengine/symengine.py

6. Tag a release in symengine/symengine-wheels

7. Upload the wheels and sdist in symengine/symengine-wheels to PyPI