Add support of reporting subtest

when one uses `pytest-subtests` plugin, this plugin was
able to report only the last report from the test, which
is passing always (if no setup/teardown issues, or failure outside
of the context of subtests), and the subtest failure were lost
and not counted/reported.

this change is fixing this, and collects all of the reports during a test
and report them each on it's own.

this introduce a new field to the output of a test report, `subtest` if it's
a report from subtest, it would be the message of that subtest.

Fixes: #28

github action: switch to using `python -m build`

move away from using setuptools, which isn't installed
by default in new version of python

fix(#17): handle case when xdist isn't installed or disabled

Closes: #17

fix: Only function markers are collected

the collection of markers wasn't using the right pytest function
to take all of the markers for an item, now we are using `iter_markers()`

Closes: #14

feature(slices): incress test coverage

fix publish of bdist_wheel

add building on tags

github actions: fix badge link

Stop testing on python3.5

Add support for Github Action data collection