FIX: Canonicalize environment dicts to strings in Windows

[effigies]

Summary

Reading through #2509 and CPython's _winapi.c, it looks like we actually want to pass dicts of str, rather than bytes.

        if (! PyUnicode_Check(key) || ! PyUnicode_Check(value)) {
            PyErr_SetString(PyExc_TypeError,
                "environment can only contain strings");
            goto error;
        }

Fixes #2509.

List of changes proposed in this PR (pull-request)

• Fix canonicalize to str
• Pass canonicalized env to Popen in version_from_command()

This last is deprecated and shouldn't be getting called, but NiftyReg still uses it, so that's a separate task.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3267 (0dbcedd) into master (35b7927) will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master    #3267   +/-   ##
=======================================
  Coverage   64.69%   64.69%           
=======================================
  Files         302      302           
  Lines       39837    39837           
  Branches     5286     5286           
=======================================
  Hits        25773    25773           
  Misses      12974    12974           
  Partials     1090     1090           

Flag	Coverage Δ
unittests	64.69% <33.33%> (ø)

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Impacted Files	Coverage Δ
nipype/utils/filemanip.py	71.46% <20.00%> (ø)
nipype/interfaces/base/core.py	84.14% <100.00%> (ø)

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