Upstream MediaWiki is trying to drop the 'writeapi' flag in the siteinfo
API output as a part of [0]. This has been always 'true' for the past
several years, but mwclient still always checks it and throws an error
if it's not there: [1]. That check should be removed from mwclient.

[0]: https://phabricator.wikimedia.org/T294397
[1]: https://phabricator.wikimedia.org/T371977

As one of the current mwclient maintainers: we have merged a patch to
remove the expectation of the writeapi flag -
https://github.com/mwclient/mwclient/pull/345 - and intend to do a
0.11.0 mwclient release today (which will also contain a whole bunch of
other changes, so for stable releases you will likely just want to
backport that patch).