I wrote my static site generator in Python 3.7 by manually translating the Perl version I had written earlier. Both are available at github https://github.com/john-bokma/tumblelog
Example of this set up: http://plurrrr.com/
Amusingly, your tumblelog.py is about the same size as dercuano.py, while the intersection of features is almost exactly empty. We managed to do even the most basic things differently — for example, yours reads a single long file with % lines separating the entries (like a fortune file!) and produces a passel of output files, while mine reads a passel of input files and produces a single multi-megabyte output file. This diversity of choices perhaps demonstrates the need for experimentation to see what works best for different people and in different circumstances. (Or it could just mean I'm insane.)
In case it's useful to someone else, as inspiration if nothing else, it's at https://gitlab.com/kragen/dercuano, in the public domain. The latest version of the output is at http://canonical.org/~kragen/dercuano-20190525.tar.gz.