routing: Fix breakages with routing on some versions of Python. Finally figured what happened with change 6baa94da8b16 (this is a Mercurial hash by the way if you're looking at the Git mirror). Between Python 3.6 and 3.7 there was a change where the percent sign ('%') went from being escaped by `re.escape` to _not_ being escaped. So now we need to use different regex patterns dependin on the Python version, yay.

---
config:
    site:
        root: /~john/public/
in:
    pages/about.html: 'URL: {{page.url}}, LINK: {{pcurl("missing")}}'
    pages/_index.html: 'URL: {{page.url}}'
outfiles:
    about.html: 'URL: /%7Ejohn/public/about.html, LINK: /%7Ejohn/public/missing.html'
    index.html: 'URL: /%7Ejohn/public/'