MAINTENANCE: Automated closing of pull requests.

This commit exists to close the following pull requests on Github:

Closes #1612 (close requested by 'marmbrus')
Closes #2723 (close requested by 'marmbrus')
Closes #1737 (close requested by 'marmbrus')
Closes #2252 (close requested by 'marmbrus')
Closes #2029 (close requested by 'marmbrus')
Closes #2386 (close requested by 'marmbrus')
Closes #2997 (close requested by 'marmbrus')