MAINTENANCE: Automated closing of pull requests.

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

Closes #3880 (close requested by 'ash211')
Closes #3649 (close requested by 'marmbrus')
Closes #3791 (close requested by 'mengxr')
Closes #3559 (close requested by 'andrewor14')
Closes #3879 (close requested by 'ash211')