rixmp install fails on CI for windows-latest py3.6 or py3.7

[khaeru]

Through #359 (comment), it was discovered that the windows-latest py3.6 check failed at the step "Install R package and dependencies", with output like:

** testing if installed package can be loaded from temporary location
##[error]Error: package or namespace load failed for 'rixmp':
ERROR: loading failed
* removing 'D:/a/_temp/Library/rixmp'
 .onLoad failed in loadNamespace() for 'rixmp', details:
  call: py_module_import(module, convert = convert)
  error: ModuleNotFoundError: No module named 'ixmp.backend'

Detailed traceback: 
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\site-packages\ixmp\__init__.py", line 6, in <module>
    from .backend import BACKENDS, ItemType

##[error]Error: loading failed
Execution halted
##[error]Error: Failed to install 'rixmp' from local:
  (converted from warning) installation of package 'C:/Users/RUNNER~1/AppData/Local/Temp/RtmpyCyTV6/file128875cf1769/rixmp_3.0.0.9000.tar.gz' had non-zero exit status
Execution halted
##[error]Process completed with exit code 1.

• A similar error was seen in a different step of this message_ix build, also for windows-latest py3.6.
• The same error occurs with Python 3.7; see here.

The temporary response was to exclude this combination from the matrix. Two possible ways to resolve this:

1. Drop support for Python 3.6 or also 3.7, maybe just on Windows, or
2. Investigate and determine why the code fails at this point.

[khaeru]

In #416 rixmp is deprecated, so probably this job can be re-enabled.