MAINTENANCE: Automated closing of pull requests.

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

Closes #1890 (requested by andrewor14, JoshRosen)
Closes #3558 (requested by JoshRosen, marmbrus)
Closes #3890 (requested by marmbrus)
Closes #3895 (requested by andrewor14, marmbrus)
Closes #4055 (requested by andrewor14)
Closes #4105 (requested by andrewor14)
Closes #4812 (requested by marmbrus)
Closes #5109 (requested by andrewor14)
Closes #5178 (requested by andrewor14)
Closes #5298 (requested by marmbrus)
Closes #5393 (requested by marmbrus)
Closes #5449 (requested by andrewor14)
Closes #5468 (requested by marmbrus)
Closes #5715 (requested by marmbrus)
Closes #6192 (requested by marmbrus)
Closes #6319 (requested by marmbrus)
Closes #6326 (requested by marmbrus)
Closes #6349 (requested by marmbrus)
Closes #6380 (requested by andrewor14)
Closes #6554 (requested by marmbrus)
Closes #6696 (requested by marmbrus)
Closes #6868 (requested by marmbrus)
Closes #6951 (requested by marmbrus)
Closes #7129 (requested by marmbrus)
Closes #7188 (requested by marmbrus)
Closes #7358 (requested by marmbrus)
Closes #7379 (requested by marmbrus)
Closes #7628 (requested by marmbrus)
Closes #7715 (requested by marmbrus)
Closes #7782 (requested by marmbrus)
Closes #7914 (requested by andrewor14)
Closes #8051 (requested by andrewor14)
Closes #8269 (requested by andrewor14)
Closes #8448 (requested by andrewor14)
Closes #8576 (requested by andrewor14)