Ignore report files created by pytest-html

It's a plugin that's not installed by default, but it proved to be very
useful when running large sets of tests, e.g. package manager unit
tests. The created report spoils the git index.

.gitignore

@@ -46,6 +46,9 @@ tmt.1
 .mypy_cache
 .pytest_cache
 *.tgz
+# Created by pytest-html reporting plugin
+/assets/style.css
+/report.html
 
 # Virtual environment
 .env