Pytest migration (#519)
* Add pytest and pytest-asyncio to the requirements
* Update aiohttp ext test
* Update setup.cfg
* Update tox.ini
* Add pytest to the tox requirements
* Update tox.ini
* Move configuration to tox.ini
* Add pytest configs
* Rename pytest-py34-py35.ini -> pytest-py35.ini
* Update config file paths
* Update makefile
* Migrate common tests to pytest
* Migrate FastAPI and Flask wiring tests
* Rename flask and fastapi wiring test files
* Move wiring autoloader tests
* Add pytest-asyncio to the tox.ini
* Migrate wiring async injection tests
* Migrate main wiring tests
* Migrate wiring string module and package names tests
* Migrate wiring config tests
* Migrate misc wiring tests
* Update tests structure
* Migrate misc wiring tests
* Refactor container.from_schema() API tests
* Migrate container.from_schema() integration tests
* Rename schema samples
* Update sample imports
* Migrate container tests
* Refactor container tests
* Migrate container self tests
* Migrate container instance tests
* Migrate container custom string attribute name tests
* Migrate container async resource tests
* Fix py2 container tests
* Migrate container cls tests
* Migrate container class custom string cls as atrribute name tests
* Migrate ext.aiohttp tests
* Migrate ext.flasks tests
* Update ext package tests doc block
* Migrate provider utils tests
* Migrate Factory async mode tests
* Migrate async tests
* Rename common test module
* Refactor asserts in provider tests
* Migrate factory tests
* Migrate selector provider tests
* Migrate object provider tests
* Migrate self provider tests
* Migrate delegate provider tests
* Migrate provider tests
* Migrate dependency provider tests
* Migrate dependencies container provider tests
* Fix warnings
* Migrate list provider tests
* Migrate dict provider tests
* Migrate callable tests
* Migrate injection tests
* Migrate container provider tests
* Migrate coroutine providers
* Migrate traversal tests
* Migrate resource tests
* Migrate configuration tests
* Migrate provided instance provider tests
* Update doc blocks and imports
* Migrate singleton tests
* Update changelog and cosmetic fixes
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