Reproc to call subprocesses cross platform (useful for micromamba)

[wolfv]

Checklist

• Title of this PR is meaningful: e.g. "Adding my_nifty_package", not "updated meta.yaml"
• License file is packaged (see here for an example)
• Source is from official source
• Package does not vendor other packages. (If a package uses the source of another package, they should be separate packages or the licenses of all packages need to be packaged)
• If static libraries are linked in, the license of the static library is packaged.
• Build number is 0
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• GitHub users listed in the maintainer section have posted a comment confirming they are willing to be listed there

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipes/reproc) and found some lint.

Here's what I've got...

For recipes/reproc:

• Selectors are suggested to take a <two spaces>#<one space>[<expression>] form. See lines [25, 45, 66, 89]
• There are 1 too many lines. There should be one empty line at the end of the file.

For recipes/reproc:

• It looks like the 'reproc-cpp' output doesn't have any tests.
• It looks like the 'reproc-cpp-static' output doesn't have any tests.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipes/reproc) and found some lint.

Here's what I've got...

For recipes/reproc:

• Selectors are suggested to take a <two spaces>#<one space>[<expression>] form. See lines [25, 45, 68, 91]
• There are 1 too many lines. There should be one empty line at the end of the file.

For recipes/reproc:

• It looks like the 'reproc-cpp' output doesn't have any tests.
• It looks like the 'reproc-cpp-static' output doesn't have any tests.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/reproc) and found it was in an excellent condition.

[wolfv]

confirmation by @DaanDeMeyer on gitter: https://gitter.im/reproc/Lobby?at=5fa577b5bf955735eb62cabb