MAINTENANCE: Automated closing of pull requests.

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

Closes #2849 (close requested by 'srowen')
Closes #2786 (close requested by 'andrewor14')
Closes #4678 (close requested by 'JoshRosen')
Closes #5457 (close requested by 'andrewor14')
Closes #3346 (close requested by 'andrewor14')
Closes #6518 (close requested by 'andrewor14')
Closes #5403 (close requested by 'pwendell')
Closes #2110 (close requested by 'srowen')