[CI] Stop pinning an old attrs version

simplesat (required by fusesoc) was broken by an update to the attrs
library. Now simplesat 0.8.2 has been released which fixes the issue, we
can remove the explicit pinning of the old attrs version.

azure-pipelines.yml

@@ -31,9 +31,6 @@ 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 \
@@ -48,7 +45,7 @@ jobs:
           flex \
           bison \
           curl \
-        && sudo pip3 install -U six attrs==19.1.0 fusesoc
+        && sudo pip3 install -U six fusesoc
     displayName: Install dependencies
 
   - bash: |