Add importlib-metadata explicitly on tests/pytest-pypi

tests/pytest-pypi/setup.py

@@ -90,7 +90,10 @@ setup(
     keywords='pytest-pypi testing pytest pypi',
     packages=find_packages(exclude=["contrib", "docs", "tests*"]),
     include_package_data = True, # include files listed in MANIFEST.in
-    install_requires = ['Flask', 'six'],
+    install_requires = [
+       'Flask',
+       'six',
+       'importlib-metadata'],
 
     # the following makes a plugin available to pytest
     entry_points = {