4.6: Update copyright year to 2020

[hugovk]

Happy new year!

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• [4.6 branch] Target the master branch for bug fixes, documentation updates and trivial changes.
• [4.6 branch] 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:

• [trivial] Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.

• Add yourself to AUTHORS in alphabetical order.

[nicoddemus]

Thanks!

I see other occurrences though:

• license.rst: https://github.com/pytest-dev/pytest/blob/4.6-maintenance/doc/en/license.rst
• epub_copyright: https://github.com/pytest-dev/pytest/blob/4.6-maintenance/doc/en/conf.py#L278

[hugovk]

Updated, plus one more.

[nicoddemus]

Thanks!