Do not pin sphinx version as it gets replaced on RTD

See readthedocs/readthedocs.org#3829

Author: sebp

environment.yml

@@ -4,9 +4,10 @@ channels:
 dependencies:
   - python=3.7
   - gettext
+  - mock
+  - pillow
   - pip
-  - sphinx==1.7.6
-  - sphinx_rtd_theme>=0.3.1
+  - sphinx
+  - sphinx_rtd_theme
   - pip:
-    - sphinx-autobuild==0.7.1
     - sphinx-intl