Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduced "require" fixture to provide per-test requirements check.
To make fixtures accessible inside of setup methods in tests all these methods
are renamed to the "setup_method_fixture" and decorated by autouse flag.
Also all setup methods moved to the top of the files.
Previously, it was necessary to support older versions of Python for
compatibility. F-strings were released in Python 3.6. Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.
To avoid closing the body fd prematurely, the fd value is moved from
the request struct to the app link. The body fd should not be closed
immediately after the request is sent to the application due to possible
request rescheduling.
