Black line length doesn't match default flake8 length

I've noticed that the default black line length of 88 doesn't match that of flake8 (79).

I would suggest setting the default flake8 length to 88. See https://github.com/psf/black

```
--- a/segment_proxy/.flake8
+++ b/segment_proxy/.flake8
@@ -1,3 +1,4 @@
[flake8]
-max-line-length = 79
+max-line-length = 88
exclude = env/*
+extend-ignore = E203
```