httpie: use python3 by default

This package is intended to be used as application and supports
python3[1] (and is about to deprecated python2.7 support[2]),
so there's no reason to not use it in 2019.

[1] https://github.com/jakubroztocil/httpie/tree/1.0.3#python-version
[2] https://github.com/jakubroztocil/httpie/commit/b3d2c1876e23e7de7afe588e4f64160d17201b45