Install pytest into the virtual environment

This follows up to commit 018499ec, which introduced a pytest import.

Makefile

@@ -110,7 +110,7 @@ all-redacted:
 # it's one less thing for a user to think about.
 venv:
 	virtualenv -p python3 venv
-	venv/bin/pip3 install python-frontmatter jinja2 click
+	venv/bin/pip3 install python-frontmatter jinja2 click pytest
 
 # LaTeX litters a lot
 clean_latex: