Add pre-commit and configure hook to review with Black

Disclaimer:
Participation by NIST in the creation of the documentation of mentioned
software is not intended to imply a recommendation or endorsement by the
National Institute of Standards and Technology, nor is it intended to
imply that any specific software is necessarily the best available for
the purpose.

References:
* [AC-215] Evaluate pre-commit usage on casework repositories
* [AC-216] Apply Black to all casework Python code bases
* #37

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

.gitignore

@@ -2,6 +2,7 @@
 *.swp
 .DS_Store
 .idea/
+.venv-pre-commit/
 __pycache__
 build/
 case_utils.egg-info/

Makefile

@@ -21,7 +21,8 @@ $(error Unable to determine CASE version)
 endif
 
 all: \
-  .ontology.done.log
+  .ontology.done.log \
+  .venv-pre-commit/var/.pre-commit-built.log
 
 .PHONY: \
   download
@@ -64,8 +65,32 @@ all: \
 	test -r case_utils/ontology/case-$(case_version)-subclasses.ttl
 	touch $@
 
+# This virtual environment is meant to be built once and then persist, even through 'make clean'.
+# If a recipe is written to remove this flag file, it should first run `pre-commit uninstall`.
+.venv-pre-commit/var/.pre-commit-built.log:
+	rm -rf .venv-pre-commit
+	test -r .pre-commit-config.yaml \
+	  || (echo "ERROR:Makefile:pre-commit is expected to install for this repository, but .pre-commit-config.yaml does not seem to exist." >&2 ; exit 1)
+	$(PYTHON3) -m venv \
+	  .venv-pre-commit
+	source .venv-pre-commit/bin/activate \
+	  && pip install \
+	    --upgrade \
+	    pip \
+	    setuptools \
+	    wheel
+	source .venv-pre-commit/bin/activate \
+	  && pip install \
+	    pre-commit
+	source .venv-pre-commit/bin/activate \
+	  && pre-commit install
+	mkdir -p \
+	  .venv-pre-commit/var
+	touch $@
+
 check: \
-  .ontology.done.log
+  .ontology.done.log \
+  .venv-pre-commit/var/.pre-commit-built.log
 	$(MAKE) \
 	  PYTHON3=$(PYTHON3) \
 	  --directory tests \