Update CHANGELOG and README after 0.7.1 release

[RMeli]

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[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.38%. Comparing base (d187c66) to head (c943e2a).
Report is 7 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1252   +/-   ##
=======================================
  Coverage   94.38%   94.38%           
=======================================
  Files         154      154           
  Lines        9374     9374           
  Branches     1160     1160           
=======================================
  Hits         8848     8848           
  Misses        320      320           
  Partials      206      206           

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[RMeli]

PR #1249 has been removed from the version_7.0 branch (thanks @msimberg), so that the changes for 0.7.2 do not pollute master (the release was removed).

[msimberg]

test_reduce timed out again on this branch: #603 (comment).