Upgrade development environment to Python 3.7

Last year I tried jumping to the most recent Python; the result was a
disaster.  But let’s try at least jumping to 3.7, so that I can switch
to `uv python install` instead of the old `pyenv`.  Fingers crossed.
The `jinja2` downgrade is to avoid the error:

    Could not import extension sphinx.builders.latex
    (exception: cannot import name 'contextfunction' from 'jinja2'...)

.python-version

@@ -1 +1 @@
-3.6.8
+3.7.9

requirements.txt

@@ -1,6 +1,7 @@
 astropy==3.2.2
 beautifulsoup4==4.6.0
 html5lib==1.0.1
+jinja2==3.0.3
 lxml==4.9.1
 mock==2.0.0
 numpy==1.15.4