try installing the docs environment using pip to avoid build timeouts

Related:
* https://github.com/rtfd/readthedocs.org/issues/4778
* https://docs.readthedocs.io/en/latest/guides/build-using-too-many-resources.html

docs/environment.yml

@@ -5,18 +5,19 @@ channels:
 
 dependencies:
   - python=3.6
-  - sortedcontainers
-  - scipy
-  - holoviews
-  - ipyparallel
-  - distributed
-  - ipykernel>=4.8*
-  - jupyter_client>=5.2.2
-  - ipywidgets
-  - scikit-optimize
-  - nbconvert>=5.4*  # because my jupyter-sphinx branch depends on it
   - pip:
-      - sphinx_rtd_theme
-      - git+https://github.com/basnijholt/jupyter-sphinx.git@widgets_execute
-      - git+https://github.com/python-adaptive/adaptive.git@master  # temporary solution because jupyter-sphinx doesn't use the adaptive from the repo
-      - sphinx_fontawesome
+    - sortedcontainers
+    - scipy
+    - holoviews
+    - bokeh
+    - ipyparallel
+    - distributed
+    - ipykernel>=4.8*
+    - jupyter_client>=5.2.2
+    - ipywidgets
+    - scikit-optimize
+    - nbconvert>=5.4*  # because my jupyter-sphinx branch depends on it
+    - sphinx_rtd_theme
+    - git+https://github.com/basnijholt/jupyter-sphinx.git@widgets_execute
+    - git+https://github.com/python-adaptive/adaptive.git@master  # temporary solution because jupyter-sphinx doesn't use the adaptive from the repo
+    - sphinx_fontawesome