Ban coverage>=6.3

See nedbat/coveragepy#1312. We can revert this
commit once a newer version of coverage fixes this issue.

coverage==6.3.1
(nedbat/coveragepy#1310 (comment))
doesn' t fully fix the issue either; hangs become less frequent but are
not eliminated (see e.g. this run of our tests with coverage==6.3.1:
https://github.com/cleanlab/cleanlab/runs/5036277812?check_suite_focus=true).

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
           python-version: ${{ matrix.python }}
       - name: Install dependencies
         run: |
-          pip install pytest pytest-cov
+          pip install pytest pytest-cov 'coverage<6.3'
           pip install -r requirements.txt
       - name: Install torch and torchvision
         if: "${{ !(contains(matrix.os, 'windows') && matrix.python == '2.7') }}"