[issue8513] subprocess: support bytes program name (POSIX)

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

> If the creation of os.environb is accepted (#8603), I think that
> subprocess should also be modified to support pure bytes environ. 

I fixed #8603 and opened #8640 for subprocess and envb.

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