CI: Unbreak build after simplesat breakage

Tracked in https://github.com/enthought/sat-solver/issues/270

azure-pipelines.yml

@@ -31,6 +31,9 @@ jobs:
   # Installing six is a workaround for pip dependency resolution: six is already
   # installed as system package with a version below the required one.
   # Explicitly installing six through pip gets us a supported version.
+  #
+  # attrs is correctly declared as dependency, but currently broken.
+  # Tracked in https://github.com/enthought/sat-solver/issues/270
   - bash: |
       sudo apt-get install -y \
           python3 \
@@ -45,7 +48,7 @@ jobs:
           flex \
           bison \
           curl \
-        && sudo pip3 install -U six fusesoc
+        && sudo pip3 install -U six attrs==19.1.0 fusesoc
     displayName: Install dependencies
 
   - bash: |