Header Links and Table of Contents don't build with mistune 2.x

[dairiki]

The pre-release branch of Lektor (3.4.x) installs mistune 2.x by default. (The stable 3.3 branch pins mistune to 0.8.*.)

Here, we use the lektor-markdown-header-anchors plugin to generate table of contents on documentation pages. That appears to be broken under mistune 2. Pinning mistune<2 (which current results in using mistune==0.8.4 works around the issue.

I've tested with lektor-markdown-header-anchors versions 0.1 (which is currently what our .lektorproject specifies) and 0.3.1 (the latest release).

Reported at lektor/lektor-markdown-header-anchors#14, but also reporting here as a reminder that things here are currently broken under Lektor 3.4.