tools/backport_pr: add tox.ini

Adapt tox file from `compile_and_test_for_board.py`.

`tox` is still not working without errors but gives way to start
refactoring.

dist/tools/backport_pr/.gitignore

+# tox envs directory
+.tox

dist/tools/backport_pr/tox.ini

+[tox]
+envlist = test,lint,flake8
+skipsdist = True
+
+[testenv]
+basepython = python3
+deps = -r {toxinidir}/requirements.txt
+setenv =
+    script = backport_pr.py
+commands =
+    test:       {[testenv:test]commands}
+    lint:       {[testenv:lint]commands}
+    flake8:     {[testenv:flake8]commands}
+
+[testenv:test]
+deps =
+    pytest
+    {[testenv]deps}
+commands =
+    pytest -v --doctest-modules {env:script}
+
+[testenv:lint]
+deps =
+    pylint
+    {[testenv]deps}
+commands =
+    pylint {env:script}
+
+[testenv:flake8]
+deps = flake8
+commands =
+    flake8 --max-complexity=10 {env:script}