Pin CI tests to virtualenv version 20

An issue with newer versions of `virtualenv` breaks `hatch` runs in
Python 3.8 and 3.9.

See: pypa/hatch#2193

Author: unflxw

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - name: "Install hatch"
-        run: "pip install hatch"
+        run: "pip install hatch \"virtualenv<21.0.0\""
       - name: "Run tests"
         run: "hatch -v run test.py$(echo ${{ matrix.python-version }} | tr -d '.'):pytest"