MAINTENANCE: Automated closing of pull requests.

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

Closes #3137 (close requested by 'marmbrus')
Closes #3362 (close requested by 'marmbrus')
Closes #2979 (close requested by 'JoshRosen')
Closes #2223 (close requested by 'JoshRosen')
Closes #2998 (close requested by 'marmbrus')
Closes #3202 (close requested by 'marmbrus')
Closes #3079 (close requested by 'marmbrus')
Closes #3210 (close requested by 'marmbrus')
Closes #2764 (close requested by 'marmbrus')
Closes #3618 (close requested by 'marmbrus')
Closes #3501 (close requested by 'marmbrus')
Closes #2768 (close requested by 'marmbrus')
Closes #3381 (close requested by 'marmbrus')
Closes #3510 (close requested by 'marmbrus')
Closes #3703 (close requested by 'marmbrus')
Closes #2543 (close requested by 'marmbrus')
Closes #2876 (close requested by 'marmbrus')
Closes #1281 (close requested by 'JoshRosen')

Author: pwendell