[WIP] Update version in to 0.8.1dev and release file

[rflamary]

Types of changes

Preparing release 0.8.1

Motivation and context / Related issue

How has this been tested (if it applies)

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document.
• All tests passed, and additional code has been covered with new tests.

[codecov]

Codecov Report

Merging #323 (cbf076f) into master (f8d871e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #323   +/-   ##
=======================================
  Coverage   93.03%   93.03%           
=======================================
  Files          21       21           
  Lines        5198     5198           
=======================================
  Hits         4836     4836           
  Misses        362      362