ignore errors from flake8-black (#690)

we already run black in its own pre-commit hook and on CI,
so no need to also have flake8/hound run black.

Author: beniwohli

.flake8

@@ -1,4 +1,4 @@
 [flake8]
 exclude=elasticapm/utils/wrapt,build,src,tests,dist,conftest.py,setup.py
 max-line-length=120
-ignore=E731,W503,E203
+ignore=E731,W503,E203,BLK100