Add the readthedocs dependencies to environment.yml

Potentially a solution to;
 - readthedocs/readthedocs.org#6870

Hopefully fixes chipsalliance#43.

Signed-off-by: Tim 'mithro' Ansell <[email protected]>

Author: mithro

docs/environment.yml

@@ -11,5 +11,11 @@ dependencies:
 - pygobject
 - yosys
 - netlistsvg
-- pip:                            # Packages installed from PyPI
+# ReadTheDoc dependencies
+- mock
+- pillow
+- sphinx
+- sphinx_rtd_theme
+# Packages installed from PyPI
+- pip:
   - -r file:requirements.txt