Align black line length for tox with precommit

.pre-commit-config.yaml passes --line-length=79 to black, while
tox.ini was passing -l 78, which left five files flipping back
and forth between local and github checks.

Those five files conform to a 79-character length, so change
the local check to match the GitHub check.

tox.ini

@@ -27,7 +27,7 @@ deps =
     {[testenv:flake8]deps}
     black
 commands =
-    black -l 78 {env:BLACK_ARGS:} -t py37 src/rfc3986 tests/
+    black -l 79 {env:BLACK_ARGS:} -t py37 src/rfc3986 tests/
     {[testenv:flake8]commands}
 
 [testenv:flake8]