Bump importlib-metadata from 0.17 to 0.18

Bumps [importlib-metadata](http://importlib-metadata.readthedocs.io/) from 0.17 to 0.18.

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

test-requirements.txt

@@ -18,7 +18,7 @@ entrypoints==0.3          # via flake8
 flake8==3.7.7
 idna==2.8
 immutables==0.9
-importlib-metadata==0.17  # via pluggy, pytest
+importlib-metadata==0.18  # via pluggy, pytest
 ipython-genutils==0.2.0   # via traitlets
 ipython==7.5.0
 isort==4.3.20             # via pylint