tox.ini: Avoid using bash where unnecessary

tox.ini: Avoid using bash where unnecessary

Switch to sh, which is hopefully more ubiquitously available than bash.

A recent change (commit 085a35d657cf0fa41a402f2af66c4beaa0f60db2)
introduced bash dependency for "tox -e pep8".  It broke my environment,
where bash is not available.  This change aims to restore it.
As far as I undestand, the change in question doesn't actually need
the specific shell dialect.  So switch to sh, which is expected to be
available for any POSIX-like systems, would improve the situation.

Closes-Bug: #1382327
Change-Id: Ie023d2265d9949a3e3721f03e865e4acfb460de2