Add tests on each platform for the wheels

Author: rhpvorderman

.github/workflows/ci.yml

@@ -155,6 +155,8 @@ jobs:
           CIBW_BUILD: "cp3{6,7,8,9}-*"
           CIBW_SKIP: "*-win32 *-manylinux_i686"  # Skip 32 bit.
           CIBW_MANYLINUX_X86_64_IMAGE: "manylinux2014"
+          CIBW_TEST_REQUIRES: pytest
+          CIBW_TEST_COMMAND: pytest {project}/tests
       - name: Build sdist
         if: "runner.os == 'Linux'"
         run: python setup.py sdist