.PHONY: all
all: tests

.PHONY: tests
tests: py_env
    bash -c 'source py_env/bin/activate && py.test tests'

py_env: requirements_dev.txt setup.py
    rm -rf py_env
    virtualenv py_env
    bash -c 'source py_env/bin/activate && pip install -r requirements_dev.txt'

# Makefile
.PHONY: all
all: tests

.PHONY: tests
tests:
    tox

# tox.ini
[tox]
project = my_project
envlist = py26,py27

[testenv]
install_command = pip install --use-wheel {opts} {packages}
deps = -rrequirements_dev.txt
commands =
    py.test {posargs:tests}

REBUILD_FLAG =

.PHONY: all
all: tests

.PHONY: tests
tests: .venv.touch
    tox $(REBUILD_FLAG)

.venv.touch: setup.py requirements.txt requirements_dev.txt
    $(eval REBUILD_FLAG := --recreate)
    touch .venv.touch

$ make tests
touch .venv.touch
tox --recreate
[[ SNIP ]]
$ make tests
tox
[[ SNIP ]]
$ touch requirements.txt
$ make tests
touch .venv.touch
tox --recreate
[[ SNIP ]]