[CI] Azure.sonic-swss (TestAsan vstest) Install dependencies: pip3 not found

[peterbailey-arista]

Azure pipelines builds are failing on test vstest at the install dependencies step

• sudo pip3 install pytest flaky exabgp docker lcov_cobertura
  sudo: pip3: command not found

seen in many or all recent PRs:
#4749
#4638

similar to closed issue:
#4763

[vmittal-msft]

@peterbailey-arista fix : sonic-net/sonic-swss-common#1226

[yijingyan2]

Hi @peterbailey-arista, this issue has been fixed by #4761.
The failed ones ran against the old code base without this fix commit, /azpw retry won't pull the latest target branch. I have triggered fresh runs for these two PRs. Thanks