Rename pyproject.toml to .black.toml to address pep517

Tox has brought to our attention that using pyproject.toml along
with setup.py/setup.cfg is confusing.

Since the only thing we're using it for is to configure black,
rename the file and use it as a black-specific configuration file
to address the confusion.

Fixes: https://github.com/ansible-community/ara/issues/33
Change-Id: I8d03927cefcfe076aa88ca6b87d45f055de9a4f4

tests/linters.sh

@@ -40,7 +40,7 @@ if [ -z "${VIRTUAL_ENV}" ]; then
 fi
 
 banner black
-time black --diff --check "${PROJECT_LIB}"
+time black --config ${PROJECT_ROOT}/.black.toml --diff --check "${PROJECT_LIB}"
 ret+=$?
 
 banner isort