MAINTENANCE: Automated closing of pull requests.

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

Closes #1480 (close requested by 'pwendell')
Closes #4205 (close requested by 'kdatta')
Closes #4114 (close requested by 'pwendell')
Closes #3382 (close requested by 'mengxr')
Closes #3933 (close requested by 'mengxr')
Closes #3870 (close requested by 'yhuai')