Update Makefile rules (#6996)

Makefile

@@ -2,17 +2,14 @@
 init:
 	python -m pip install -r requirements-dev.txt
 test:
-	# This runs all of the tests on all supported Python versions.
-	tox -p
+	python -m pytest tests
+
 ci:
 	python -m pytest tests --junitxml=report.xml
 
 test-readme:
 	python setup.py check --restructuredtext --strict && ([ $$? -eq 0 ] && echo "README.rst and HISTORY.rst ok") || echo "Invalid markup in README.rst or HISTORY.rst!"
 
-flake8:
-	python -m flake8 src/requests
-
 coverage:
 	python -m pytest --cov-config .coveragerc --verbose --cov-report term --cov-report xml --cov=src/requests tests