Menton python module 'pexpect' that is required for tests

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Use versioned python binaries

Because of v2->v3 migration mess different systems have different python
set by default. It is recommended to use versioned binaries.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>