add ability to remove base crypto backend for tests

commands_pre requires tox >= 3.4.0

Author: mattsb42-aws

tox.ini

@@ -1,5 +1,8 @@
 [tox]
-envlist = py{27,34,35,36,py}-{base,cryptography,pycryptodome,pycrypto,compatibility},flake8
+minversion = 3.4.0
+envlist =
+    py{27,34,35,36,py}-{base,cryptography-only,pycryptodome,pycrypto,compatibility},
+    flake8
 skip_missing_interpreters = True
 
 [testenv:basecommand]
@@ -19,6 +22,9 @@ deps =
     pytest-cov
     pytest-runner
     compatibility: {[testenv:compatibility]deps}
+commands_pre =
+    # Remove the python-rsa backend
+    only: pip uninstall -y ecdsa rsa
 commands =
     # Test the python-rsa backend
     base: {[testenv:basecommand]commands} -m "not (cryptography or pycryptodome or pycrypto or backend_compatibility)"