Gssapi

* Support for running tests against Heimdal in CI (https://github.com/pythongssapi/python-gssapi/pull/262)
* Add Kerberos specific GSS-API Extensions (https://github.com/pythongssapi/python-gssapi/pull/261)
* Tidy up docs and turn warnings into errors (https://github.com/pythongssapi/python-gssapi/pull/260)
* Support DCE IOV functions on macOS (https://github.com/pythongssapi/python-gssapi/pull/258)

- Support for 32-bit Windows python (248, 249)
- Enable specification of krb5-config via environment variable (253, 250)
- Support for GSS_C_CHANNEL_BOUND_FLAG (251)
- Support for docs build with sphinx 4 (246)
- Fix undefined variable in get_all_statuses() (241, 245)
- Add support for str objects in the cred store dict (242, 243)

Note that due to issues with our release tooling for 32-bit Windows, there effectively was no v1.6.13 release.

- Fix GSSAPI detection for macOS Big Sur (235, 236)
- Clarify how to disable detection (234)