asyncssh: loosen upper version limit

[pmrowla]

see #17

[codecov-commenter]

Codecov Report

Merging #19 (12ee72f) into master (6a71778) will not change coverage.
The diff coverage is n/a.

❗ Current head 12ee72f differs from pull request most recent head 19bccb8. Consider uploading reports for the commit 19bccb8 to get more accurate results

@@           Coverage Diff           @@
##           master      #19   +/-   ##
=======================================
  Coverage   94.17%   94.17%           
=======================================
  Files          13       13           
  Lines         670      670           
=======================================
  Hits          631      631           
  Misses         39       39           

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