Building the documentation does not work. Remove numpy from the dependencies.

CsatiZoltan · CsatiZoltan · commit a2b4213d7724 · 2020-09-08T19:08:15.000+02:00
readthedocs/readthedocs.org#1639 (comment)
diff --git a/requirements.txt b/requirements.txt
@@ -1,4 +1,3 @@
-numpy~=1.18.1
 matplotlib~=3.1.3
 scikit-image~=0.16.2
 skan~=0.8
