1.21.1

FIXED: Refer to permission_classes as a tuple (#678)
IMPROVED: Document drf-extra-fields base64 integration (#445)
ADDED: Added many support to example code (#695)
ADDED: Allow specifying response as a reference (#757)
FIXED: Fix old spelling errors and add a cspell configuration (#796)
FIXED: Remove universal wheel, python 2 is unsupported (#782)
FIXED: Fix duration field inspector (#549)