Change py2-style print stmts to py3

[cphyc]

PR Summary

Many docstrings as well as cython code were still using Python 2-style prints (without parenthesis). This PR fixes this.

It relies on the following sed expression

sed -i 's/print \([^(].*[^\\]$/print(\1)/g' **/*.{py,pyx,pxd}

with some human-based post-processing. Indeed, this regexp is unable to find multi-line print statement, which are found in particular in Cython files.

PR Checklist

• Code passes flake8 checker

[munkm]

Thanks for getting these!