Updated pre-commit config to use 19.10b0 version of Black

The latest "stable" version of Black formats code
differently from the 19.10b0 version that has been
used in the past, which introduces unwelcome
formatting changes in newer commits.
This commit explicitly sets the revision to
19.10b0.
It also changes the repo parameter to point to the
new URL.

Signed-off-by: Dwight Lidman <dwight.lidman@arm.com>
Change-Id: I20c73f0c87434143f62282f5f0399f73cedfd6ce
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index d480f6e..55a51a5 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -5,8 +5,8 @@
     hooks:
     -   id: reorder-python-imports
 
--   repo: https://github.com/ambv/black
-    rev: stable
+-   repo: https://github.com/psf/black
+    rev: 19.10b0
     hooks:
     -   id: black
         language_version: python3.6