bring the appveyor build to life again

[ppk42]

I suspect that an appveyor build is started by a webhook on Github (or by the pull request).
I see changing the appveyor.yml with a subsequent pull request as the only way to isolate the error and to try out ideas.

[ppk42]

I found a miniconda environment that does not fail for conda update. But now there are errors with the Python 3.7 build.

I inspected the code: Tests use a private method from the _AxesBase class in the matplot lib. This method hat two keyword params xdata and ydata. The signature of the method changes from version 3.3.4 (used for the Python 3.6 build) to version 3.4.3 for python 3.7, 3.8 and 3.9 builds.

In matplot 3.4.3 the keyword arguments have been removed. Python 3.7 raises an error because of the missing arguments. Higher Python version do not report an error, even though the wrong method signature was used too. This is dagenrous and until we find a better solution I sugesst to freez the matplotlib version.

I'll try to freeze the matplolib version and also the scipy and numpy version. For a check I need to perform the PR again. I'll mark all changes in the setup.py and appveyor.yml with the corresponding issue number. #955

[ppk42]

Frezing the numpy and scipy version was a bad idea. The 3.5 build crashes on that. So I decided to use a conditional aproach for matplotlib only (differnt for 3.5 at one side and all above 3.6 at the other).

[codecov]

Codecov Report

Merging #956 (a93c4ac) into devel (3e19a85) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel     #956      +/-   ##
==========================================
- Coverage   54.59%   54.54%   -0.05%     
==========================================
  Files         211      210       -1     
  Lines       21564    21527      -37     
  Branches     2891     3163     +272     
==========================================
- Hits        11772    11742      -30     
+ Misses       9240     9232       -8     
- Partials      552      553       +1     

[ppk42]

Gehbiszumeis has adopted my changes in his pull request (#951). The explicit merging of this pull request is not necessary. For documentation reasons, I will not delete the PR, but it can be closed.

[ppk42]

Of cause, if for any reason #951 will not be accepted then, the appveyor build will stay broken.