Pin Black to a version that does not require gcc

Recent versions of black require gcc to build a new regex dependency.
This may go away, so we'll just pin to a specific version black (which
is best practice anyway).

Related to psf/black#1112.

Author: mmlb

.drone.yml

@@ -10,7 +10,7 @@ pipeline:
   fmt_and_lint:
     image: python:3.6-alpine
     commands:
-      - pip install black pylama
+      - pip install black==19.3b0 pylama
       - black --check --diff .
       - pylama packet test setup.py