ci: use python 3 in reusable_testing.yml workflow

[ylobankov]

The memcached tests have migrated to Python 3 recently in scope of #82.
But reusable_testing.yml workflow wasn't updated and tests started to
fail. This patch fixes the issue.

[ligurio]

#82 introduced Python 3 without breaking compatibility with Python 2, so it should work in general.

There is a typo in commit message "This path".

LGTM

[ylobankov]

#82 introduced Python 3 without breaking compatibility with Python 2, so it should work in general.

There is a typo in commit message "This path".

LGTM

Typo is fixed, thanks!

In reusable_testing.yml workflow, we set up python2 for testing, but the test-run version was updated in #83 and now tests are run via python3. So we had missing dependencies because all of them were installed in python2 env.

[Totktonada]

LGTM.

Am I understand right, the problem is just in shebang in the main test-run script?

[ylobankov]

LGTM.

Am I understand right, the problem is just in shebang in the main test-run script?

Yes, you're right.

[Totktonada]

It is the weird story.