Upgrade dev environment to Python 3.8

A bump from 3.7 to 3.8 might sound trivial, but it took at least a dozen
attempts at tweaking `requirements.txt` before it worked out.  Each time
I had to start over from the beginning.  Turns out?  I needed to pin
`pygments` to avoid massive changes to the documentation HTML, which I
don’t have time to vet right now.

Author: brandon-rhodes

.python-version

@@ -1 +1 @@
-3.7.9
+3.8.16

requirements.txt

@@ -1,10 +1,10 @@
-astropy==3.2.2
+astropy==4.3.1
 beautifulsoup4==4.6.0
 html5lib==1.0.1
 jinja2==3.0.3
 lxml==4.9.1
 mock==2.0.0
-numpy==1.15.4
+numpy==1.17.5
 matplotlib==3.3.0
 pandas==1.0.0
 pyflakes==2.1.1