Switch back to a venv for docs + fix .circle config

.circleci/config.yml

@@ -104,7 +104,7 @@ jobs:
                 name: install dependencies
                 command: |
                     pip install sphinx
-                    pip install -r docs/doc-requirements.txt
+                    pip install -r docs/requirements.txt
 
             - save_cache:
                 paths:

docs/environment.yml

@@ -1,9 +0,0 @@
-name: tljh
-channels:
-  - conda-forge
-dependencies:
-- python=3.6
-- traitlets>=4.1
-- sphinx>=1.4, !=1.5.4
-- pip:
-  - sphinx_copybutton

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx>=1.4, !=1.5.4
+sphinx_copybutton