Postprocess coverage index.html with python instead

This change jettisons the awk and ed dependencies and instead uses a real HTML parser, via the BeautifulSoup library in python.

Using BeautifulSoup lets us do trickier stuff, like actually extracting the coverage totals and adding our own table, with per-crate coverage.

The script only does this post-processing when it finds python3; the script exits with an error if BeautifulSoup isn't installed.

Closes #249 (closed)