pin pyviz_comms=0.7.2

HoloViews didn't have a strict pinning for it.
In the new version https://github.com/holoviz/pyviz_comms/blob/v0.7.2/pyviz_comms/__init__.py#L102
was removed, while holoviews=1.11.3 seems to depend on it.

Author: basnijholt

docs/environment.yml

@@ -14,6 +14,7 @@ dependencies:
   - ipywidgets=7.4.2
   - jupyter_sphinx=0.2.2
   - atomicwrites=1.3.0
+  - pyviz_comms=0.7.2
   - pip:
     - sphinx_fontawesome==0.0.6
     - m2r==0.2.1