require {SPHINX,BREATHE}_VERSION to have == in the env var

tox.ini

@@ -1,33 +1,17 @@
 [tox]
 envlist = py, flake8
 
-# TODO: is there a way to just somehow do
-# deps = sphinx if NOT env:SPHINX_VERSION
-#               else sphinx==SPHINX_VERSION
-# ?
-# I couldn't figure out how to conditionally exclude the == signs without making
-# the environment variable SPHINX_VERSION='==X.Y.Z' which is ridiculous.
-#
-# Problem right now is I need to remember to update this variable...
-[sphinx]
-latest_version = 2.0.1
-
-[breathe]
-latest_version = 4.13.0.post0
-
 [testenv]
-# Required to be able to send coverage reports from Travis to codecov
-passenv = PYTHONWARNINGS TOXENV CI TRAVIS TRAVIS_*
+passenv = PYTHONWARNINGS TOXENV CI
 # See TODO below, skipping install since I don't know what is going on...
 # usedevelop = true
 skip_install = true
-# TODO: having issues supporting breathe==4.12.0 and sphinx==1.8.5, on CI it
-# ends up getting breathe==4.13.0.post0 somehow when using -r requirements.txt.
-# Moving requirements here for the time being, will revert when 1.x is in
-# progress / dependencies are changed.
+# NOTE: environment variables *MUST* include the `==`, e.g.,
+#       SPHINX_VERSION="==3.0.0" tox -e py
+# This way, if the variable is not set, it uses the latest version.
 deps =
-    sphinx=={env:SPHINX_VERSION:{[sphinx]latest_version}}
-    breathe=={env:BREATHE_VERSION:{[breathe]latest_version}}
+    sphinx{env:SPHINX_VERSION:}
+    breathe{env:BREATHE_VERSION:}
     bs4
     lxml
     six