Python 2 is officially unsupported after 2020, which is before the expected end-of-life
date of the next round of long-term-stable distributions like Debian 10 and Ubuntu 18.04,
so those distributions are encouraging all software that can migrate to Python 3 to do so.

The down side of this is that it would make it harder to use the `rst` plugin on
very old OS releases, or on OSs where Python 3 is available but doesn't have a `python3`
symbolic link (if such OSs exist - [PEP 394](https://www.python.org/dev/peps/pep-0394/)
says they shouldn't), or in shared hosting environments where Python 2 is installed but
Python 3 isn't. (Mitigation: switching it to `python` or `python2` is a 1-line change.)

For today's upload to Debian, I switched the `#!` line in the [[plugins/rst]] plugin
to `#!/usr/bin/python3`. In upstream ikiwiki it would probably be more appropriate
to use `#!/usr/bin/env python3`, and it would certainly be more appropriate to do it
by changing the source rather than by using `sed` after installation. I didn't apply that
change upstream immediately to give other maintainers a chance to comment. Thoughts?

--[[smcv]]

> I can attest as a pkgsrc developer, where we try to build and package
> software on all sorts of platforms (some old and wacky), that as long
> as the relevant Pythons build on those platforms (and we tend to make
> sure they do), I don't foresee any negative impact of your suggested
> change to ikiwiki. Can't attest for other situations, but am generally
> biased toward biting future bullets as early as possible.
> --[[schmonz]]

>> Nobody objected, so it's now python3 for the next release. [[done]] --[[smcv]]
