Autogeneration of flint header files for SageMath

Instructions

1. Obtain a clone of the flint repo, eg git clone https://github.com/flintlib/flint2

2. Checkout to the appropriate commit, eg git checkout v2.9.0

3. Manually adjust the content of types.pxd.template (which will be used to generate types.pxd)

4. Manually adjust the content of flint_pxd_autogen/env.py

5. Run the autogen.py script, eg python autogen.py

6. Copy all the files generated in pxd_headers inside the sage source tree (ie SAGE_SRC/sage/libs/flint)