Failure due to overly strict reticulate error test

[richfitz]

     > test_check("stevedore")
     -- 1. Failure: python_locate_version (@test-httppipe-unix.R#97) ---------------
     `python_locate_version("nosuchmodule")` threw an error with unexpected message.
     Expected match: "Did not find required python module 'nosuchmodule'"
     Actual message: "lexical error: invalid char in json text.\n NA\n (right here) ------^\n"
     Backtrace:
     1. testthat::expect_error(...)
     6. stevedore:::python_locate_version("nosuchmodule")
     7. reticulate::py_discover_config(module)
     8. reticulate:::python_conda_versions()
     10. reticulate::conda_list(conda = conda)
     11. jsonlite::fromJSON(conda_envs)
     12. jsonlite:::parse_and_simplify(...)
     13. jsonlite:::parseJSON(txt, bigint_as_char)
     14. jsonlite:::parse_string(txt, bigint_as_char)

Affecting CRAN with recent reticulate. Note that the new reticulate also offers a way of specifying dependencies (https://stat.ethz.ch/pipermail/r-package-devel/2020q1/004821.html) though practically we should fix the windows support #1 properly at some point...