Ungroup and sort test_requirements.txt

This is cleanup related to the previous commit. As that file grows,
it is harder to tell immediately if a particular package is in it
when not alphabetized. (The groups were also not intuitive, with
ddt listed separately from other unit test related dependencies.)

test-requirements.txt

@@ -1,14 +1,10 @@
+black
+build
+coverage[toml]
 ddt>=1.1.1, !=1.4.3
 mypy
-
-black
-
 pre-commit
-
-build
-virtualenv
-
 pytest
 pytest-cov
-coverage[toml]
 pytest-sugar
+virtualenv