Docs: update tense

[hugovk]

pytest 4.6.0 was released in June 2019.

• https://pypi.org/project/pytest/#history

• Target the master branch for bug fixes, documentation updates and trivial changes.
• [n/a] Target the features branch for new features, improvements, and removals/deprecations.
• [n/a] Include documentation when adding new features.
• [n/a] Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• [n/a] Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• [n/a] Add yourself to AUTHORS in alphabetical order;

[nicoddemus]

Thanks!