Add default for SFTPClient.listdir

[tjstum]

At runtime, it's possible to call listdir() (i.e. without arguments) and get a Sequence[str] back, due to the default argument in the implementation. However, there is no @overload that contains 0 arguments, so mypy doesn't think that it's possible.

This adds the presence of the default to the sequence-of-strings returning overload, to match the implementation

(This is a resubmit of #671 but based on develop instead of master)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (fab96ac) to head (b07a48d).
Report is 28 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop     #673     +/-   ##
==========================================
  Coverage    99.85%   99.85%             
==========================================
  Files           95      101      +6     
  Lines        27784    29283   +1499     
  Branches      2908     3085    +177     
==========================================
+ Hits         27743    29242   +1499     
  Misses          38       38             
  Partials         3        3             

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[ronf]

Thanks very much! This is now merged.

Also, thanks for pointing out the text about the branches in CONTRIBUTING.rst. In the early days of AsyncSSH, I did accept small fixes directly in the "master" branch, but it ended up making the branch history messier when it came time to move changes from "develop" to "master". I later switched to having "develop" be the default branch in GitHub and having all changes go in there first, but I never updated CONTRIBUTING.rst to reflect that change. This is now fixed.