Keep attrs 21 away from Python 3.4 on CI

The attrs release 21 is incompatible with Python 3.4 and while yanked, the
pip that would be around with Python 3.4 doesn't understand yanking and
so installs it anyway.

See python-attrs/attrs#807

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ jobs:
         include:
         - python: '3.4'
           polib: 'polib==1.0'
+          attrs: '"attrs<21"'
           os: ubuntu-18.04
         - python: '3.5'
           os: ubuntu-18.04
@@ -37,7 +38,7 @@ jobs:
         python3 -m pip install rply
     - name: run tests
       run: |
-        python3 -m pip install pytest
+        python3 -m pip install pytest ${{matrix.attrs || 'attrs'}}
         tests/run-tests -v
     - name: check docs
       run: |