Use mamba and conda-forge channel on azure pipeline

[ericpre]

The azure pipeline builds have been failing regularly in the last few months and are currently failing on windows... This was because of broken packages on anaconda defaults channels.

Progress of the PR

• Use conda-forge channel instead of defaults,
• use mamba to install packages (conda is too slow with conda-forge channels, because of the larger number of packages on conda-forge)
• update dev guide,
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• ready for review.

[codecov]

Codecov Report

Merging #2759 (d2ec3a7) into RELEASE_next_patch (6d184cd) will not change coverage.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##           RELEASE_next_patch    #2759   +/-   ##
===================================================
  Coverage               77.30%   77.30%           
===================================================
  Files                     202      202           
  Lines                   30032    30032           
  Branches                 6561     6561           
===================================================
  Hits                    23216    23216           
  Misses                   5068     5068           
  Partials                 1748     1748           

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