setup: pin cylc-flow version

[oliver-sanders]

• See meta packages cylc-admin#130

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Does not need tests (why?).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.
• No dependency changes.

[oliver-sanders]

Since this works with dev versions we can bump this after release.

[codecov-commenter]

Codecov Report

Merging #276 (ca8b3f1) into master (995b76d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   77.40%   77.40%           
=======================================
  Files          12       12           
  Lines        1009     1009           
  Branches      176      176           
=======================================
  Hits          781      781           
  Misses        197      197           
  Partials       31       31           

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