Add Blue as an optional requirement

Author: akaihola

constraints-oldest.txt

@@ -5,6 +5,7 @@
 # versions in `setup.cfg`.
 airium==0.2.3
 black==21.8b0
+blue==0.8.0
 defusedxml==0.7.1
 flake8-2020==1.6.1
 flake8-bugbear==22.1.11

setup.cfg

@@ -48,6 +48,8 @@ isort =
     isort>=5.0.1
 color =
     Pygments>=2.4.0
+blue =
+    blue>=0.8.0
 test =
     # NOTE: remember to keep `constraints-oldest.txt` in sync with these
     airium>=0.2.3