chore: fix the preview build (#165)

The preview build fails because of
readthedocs/readthedocs.org#10290,
let's pin urllib3 to workaround that.

Author: bfabio

requirements.txt

@@ -1,2 +1,3 @@
 Sphinx==4.3.1
 sphinx-press-theme==0.8.0
+urllib3==1.26.6