Make targets for a virtualenv

It's common to make a virtualenv inside of your cloned repo
to ensure that you have the right enviornment for that repository,
and .venv is a common convention for that virtualenv.

This adds targets to create and clean a .venv virtual env using
the checked-in requirements.txt file.  It also adds the .venv
to the .gitignore, and updates the README.

Author: handrews

.gitignore

@@ -4,3 +4,6 @@ jsonschema-*.txt
 relative-json-pointer.html
 relative-json-pointer.pdf
 relative-json-pointer.txt
+
+# For the Python enviornment
+.venv

Makefile

@@ -1,11 +1,11 @@
 XML2RFC ?= xml2rfc
+VENV ?= .venv
 
 OUT = \
 	jsonschema-core.html jsonschema-core.txt \
 	jsonschema-validation.html jsonschema-validation.txt \
 	relative-json-pointer.html relative-json-pointer.txt
 
-
 all: $(OUT)
 
 %.txt: %.xml
@@ -25,6 +25,16 @@ json-schema.tar.gz: $(OUT)
 	tar -czf json-schema.tar.gz --exclude '.*' json-schema
 	rm -rf json-schema
 
+venv: $(VENV)
+
+$(VENV):
+	python -m venv .venv
+	. .venv/bin/activate && python -m pip install --upgrade pip setuptools wheel
+	. .venv/bin/activate && python -m pip install -r requirements.txt
+
+venv-clean:
+	rm -rf .venv
+
 clean:
 	rm -f $(OUT) json-schema.tar.gz
 

README.md

@@ -35,7 +35,15 @@ Labels are assigned based on [Sensible Github Labels](https://github.com/Releque
     * links.json - JSON Hyper-Schema's Link Description Object meta-schema
     * hyper-schema-output.json - The recommended output format for JSON Hyper-Schema links
 
-Install "xml2rfc" using "pip" (https://pypi.org/project/xml2rfc/) and type "make" at a shell to build the .txt and .html spec files:
+The Makefile can create the necessary Python virtual environment for you:
+
+```sh
+make venv
+source .venv/bin/activate
+make
+```
+
+Or you can manually install "xml2rfc" using "pip" (https://pypi.org/project/xml2rfc/) and type "make" at a shell to build the .txt and .html spec files:
 
 ```sh
 pip install --requirement requirements.txt