Remove tox warning about missing allowlist_externals

* The warning is:

  WARNING: test command found but not installed in testenv
    cmd: /usr/bin/sh
    env: /home/nicolas/work/libyang-python/.tox/lint
  Maybe you forgot to specify a dependency? See also the
  allowlist_externals envconfig setting.

  DEPRECATION WARNING: this will be an error in tox 4 and above!

* Depending on the operating system where tox is run, some common
  system tools are located in different places. In particular:
  - ubuntu-18.04: /bin/sh
  - ubuntu-20.04: /usr/bin/sh

* The github CI workflow is running on ubuntu-latest, which it resolves
  to ubuntu-20.04 now, where "sh" is on /usr/bin/sh, making tox
  complain about not having it allow-listed

Author: nicolasmorini

tox.ini

@@ -36,6 +36,7 @@ skip_install = true
 install_command = python3 -m pip install {opts} {packages}
 whitelist_externals =
 	/bin/sh
+	/usr/bin/sh
 commands =
 	sh -ec 'python3 -m isort $(git ls-files "*.py")'
 	sh -ec 'python3 -m black -t py35 $(git ls-files "*.py")'
@@ -52,6 +53,7 @@ deps =
 	pylint>=2.5
 whitelist_externals =
 	/bin/sh
+	/usr/bin/sh
 commands =
 	sh -ec 'python3 -m black -t py35 --diff --check $(git ls-files "*.py")'
 	sh -ec 'python3 -m flake8 $(git ls-files "*.py")'