[MRG] Release 0.8.1.0 and correct doc build

[rflamary]

Types of changes

This changed version number to 0.8.1.0 and hopefully will correct the doc build on master

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #332 (6aea6b1) into master (7ad97e3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   93.03%   93.03%           
=======================================
  Files          21       21           
  Lines        5198     5198           
=======================================
  Hits         4836     4836           
  Misses        362      362