Enable Flake8 line length (aws-observability#16)

Prior to this commit, we were only linting code length, as Black linter
does not check comments. This lead to inconsistent files, and is
resolved by enabling flake8 line length rules, which checks both code
and comments

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Author: thpierce

.flake8

@@ -1,8 +1,6 @@
 [flake8]
+max-line-length = 120
 ignore =
-  # line too long, defer to black
-  E501
-
   # allow line breaks before binary ops
   W503