testing/py3-ecos: add missing build dependency on python3-dev
src/ecosmodule.c:4:10: fatal error: Python.h: No such file or directory
4 | #include <Python.h>
| ^~~~~~~~~~
compilation terminated.
src/ecosmodule.c:4:10: fatal error: Python.h: No such file or directory
4 | #include <Python.h>
| ^~~~~~~~~~
compilation terminated.