Merge branch 'ralf/opam' into 'master'

Allow compiling against "dev" version of Coq

See merge request robbertkrebbers/coq-stdpp!20
5 jobs for master in 8 minutes and 17 seconds
Status Job ID Name Coverage
  Build
passed #6029
fp-timing
build-coq.8.6.1

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passed #6028
fp-timing
build-coq.8.7.0

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  Deploy
passed #6030
opam

00:00:14

 
  Build More
passed #6032
fp-timing
build-coq.8.6

00:02:09

passed #6031
fp-timing
build-coq.8.7.dev

00:05:09