Force virtualenv and pip to output UTF-8

[AndreLouisCaron]

Fixes #1550.

This is an alternate take on #2390 that is better targetted to the specific
issue and may prove less risky overall.

After running these two commands with output redirected to a file, we
read said files with an explicit request for UTF-8 encoding. On some
cases this does not contain UTF-8 data. For example, on a French
Canadian Windows machine with non-ASCII characters in the username,
the output contains the contents of the APPDATA environment
variable, which is printed to the current code page (cp1252).

This change enables Python's "UTF-8 Mode" when running virtualenv
and pip to guarantee the output is UTF-8 everywhere.

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