Address CI failures with R/reticulate

[khaeru]

Ubuntu jobs (but not Windows or macOS) began to fail in the last few days, e.g.:
https://github.com/iiasa/ixmp/runs/2037215238?check_suite_focus=true#step:15:87

* checking tests ...
  Running ‘testthat.R’ [7s/49s]
 ERROR
Running the tests in ‘tests/testthat.R’ failed.
Complete output:
  > library(testthat)
  > test_check("rixmp")
  Loading required package: rixmp
  Loading required package: reticulate
  Loaded rixmp v3.2.0.9000. See ?rixmp for help, citation("rixmp") for use in publication.
  
  Segmentation fault (core dumped)
* checking for non-standard things in the check directory ... OK
* checking for detritus in the temp directory ... OK
* DONE

Status: 1 ERROR, 2 NOTEs
See
  ‘/home/runner/work/ixmp/ixmp/check/rixmp.Rcheck/00check.log’
for details.

Actions to take:

• If necessary, temporarily disable this step/check.
• Investigate and identify the cause. The debug commands in the pytest workflow can be used for this purpose.
• Open a PR with a fix.

[khaeru]

The latest version of reticulate, version 1.18, was released 2020-10-25 (https://cran.r-project.org/web/packages/reticulate/index.html) so a new version is probably not the cause.

[khaeru]

In #416 the rcmdcheck of rixmp is removed with the deprecation of the package, so this is no longer an issue.