Add tox integration

tox.ini:
Add tox integration for testing (linting and coverage) in virtual
environments using poetry.

tox.ini

+[tox]
+isolated_build = true
+envlist = linter,coverage
+skip_missing_interpreters = True
+
+[testenv]
+whitelist_externals = poetry
+commands =
+    poetry install
+    poetry run pytest -v tests/ -m "not integration"
+
+[testenv:coverage]
+whitelist_externals = poetry
+commands =
+    poetry install
+    poetry run coverage run
+    poetry run coverage xml
+    poetry run coverage report --fail-under=100.0
+
+[testenv:linter]
+whitelist_externals = poetry
+commands =
+    poetry install
+    poetry run isort .
+    poetry run black --check .
+    poetry run flake8
+    poetry run mypy -p repo_management -p tests