ci: pin urllib<2 to address readthedocs build failure

Problem: ReadTheDocs CI build currently fails with:

ImportError: urllib3 v2.0 only supports OpenSSL 1.1.1+,
currently the 'ssl' module is compiled with OpenSSL 1.0.2n  7 Dec 2017
See: urllib3/urllib3#2168

Workaround the issue until the builders update their OpenSSL
by pinning urllib<2.

(copied Mark's fix in flux-security)

Co-authored-by: Mark A. Grondona <[email protected]>

Author: garlick

doc/requirements.txt

@@ -1,3 +1,4 @@
 sphinx==3.4.3
 sphinx-rtd-theme>=0.5.2
 docutils>=0.14,<0.18
+urllib3<2