- Skip more tests that are unstable.

- Update to 2.9.0:
- Added mypy-compatible type annotations to all AsyncSSH
modules, and a "py.typed" file to signal that annotations are
now available for this package.
- Added experimental support for SFTP versions 4-6. While
AsyncSSH still defaults to only advertising version 3 when
acting as both a client and a server, applications can
explicitly enable support for later versions, which will be
used if both ends of the connection agree. Not all features
are fully supported, but a number of useful enhancements are
now available, including as users and groups specified by
name, higher resolution timestamps, and more granular error
reporting.
- Updated documentation to make it clear that keys from
a PKCS11 provider or ssh-agent will be used even when
client_keys is specified, unless those sources are explicitly
disabled.
- Improved handling of task cancellation in AsyncSSH to
avoid triggering an error of "Future exception was never
retrieved". Thanks go to Krzysztof Kotlenga for reporting
this issue and providing test code to reliably reproduce it.
- Changed implementation of OpenSSH keepalive handler to
improve interoperability with servers which don't expect a
"success" response when this message is sent.