CI: Pin `black` version to latest upstream release

(cherry picked from commit 41c735fe5016285a8f0ce1175c1e8c5ab8c451af)

.github/workflows/static_checks.yml

@@ -13,7 +13,7 @@ jobs:
         run: |
           sudo apt-get update -qq
           sudo apt-get install -qq dos2unix recode clang-format
-          sudo pip3 install black pygments
+          sudo pip3 install black==20.8b1 pygments
 
       - name: File formatting checks (file_format.sh)
         run: |