Switch from nose to pytest (#5282)

* Use pipstrap to install a good version of pip

* Use pytest in cb-auto tests

* Remove nose usage in auto_test.py

* remove nose dev dep

* use pytest in test_tests

* Use pytest in tox

* Update dev dependency pinnings

* remove nose multiprocess lines

* Use pytest for coverage

* Use older py and pytest for old python versions

* Add test for Error.__str__

* pin pytest in oldest test

* Fix tests for DNS-DO plugin on py26

* Work around bug for Python 3.3

* Clarify dockerfile comments

tests/letstest/scripts/test_tests.sh

@@ -10,9 +10,9 @@ LE_AUTO_SUDO="" VENV_PATH=$VENV_NAME letsencrypt/certbot-auto --debug --no-boots
 
 # change to an empty directory to ensure CWD doesn't affect tests
 cd $(mktemp -d)
-pip install nose
+pip install pytest==3.2.5
 
 for module in $MODULES ; do
     echo testing $module
-    nosetests -v $module
+    pytest -v --pyargs $module
 done