Move testing dependencies to setup.cfg

This patch moves the packages listed in `tests/requirements.txt` into
`setup.cfg`.

This is done to:

* Reduce complexity of downstream projects that wish to inherit testing
  dependencies.  E.g. currently the `tests/requirements.txt` file is
  loaded via incorporating `case-utils` as a submodule in at least two
  projects (`case_exiftool`, `case_gnu_time`).
* Expose test requirements for supply-chain review via package metadata.

The feature name is for the test requirements is `testing`, inheriting
the name from Python's `setuptools`.

This feature is added knowing that, once added, it should not be
removed, per this best practice in the PyPA guidance:

> Best practice: if a project ends up no longer needing any other
> packages to support a feature, it should keep an empty requirements
> list for that feature in its extras_require argument, so that packages
> depending on that feature don’t break (due to an invalid feature
> name).

There were no effects observed on Make-managed files.

References:
* https://github.com/casework/CASE-Implementation-ExifTool/blob/0.5.0/tests/Makefile#L43-L45
* https://github.com/casework/CASE-Implementation-GNU-Time/blob/0.7.0/tests/Makefile#L56-L58
* https://github.com/pypa/setuptools/blob/v67.6.1/setup.cfg#L42
* https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#optional-dependencies

Signed-off-by: Alex Nelson <[email protected]>

Author: ajnelson-nist

setup.cfg

@@ -34,6 +34,14 @@ console_scripts =
     case_sparql_select = case_utils.case_sparql_select:main
     case_validate = case_utils.case_validate:main
 
+[options.extras_require]
+testing =
+    PyLD
+    mypy
+    pytest
+    python-dateutil
+    types-python-dateutil
+
 [options.package_data]
 case_utils = py.typed
 case_utils.ontology =

tests/Makefile

@@ -31,8 +31,7 @@ all: \
   $(top_srcdir)/.git_submodule_init.done.log \
   $(top_srcdir)/case_utils/__init__.py \
   $(top_srcdir)/setup.cfg \
-  $(top_srcdir)/setup.py \
-  requirements.txt
+  $(top_srcdir)/setup.py
 	rm -rf venv
 	$(PYTHON3) -m venv \
 	  venv
@@ -42,14 +41,11 @@ all: \
 	    pip \
 	    setuptools \
 	    wheel
-	source venv/bin/activate \
-	  && pip install \
-	    --requirement requirements.txt
 	source venv/bin/activate \
 	  && cd $(top_srcdir) \
 	    && pip install \
 	      --editable \
-	      .
+	      .[testing]
 	touch $@
 
 all-case_utils: \