bpo-40204: Allow pre-Sphinx 3 syntax in the doc (GH-21844)

Enable Sphinx 3.2 "c_allow_pre_v3" option and disable the
c_warn_on_allowed_pre_v3 option to make the documentation compatible
with Sphinx 2 and Sphinx 3.

Author: vstinner

Doc/conf.py

@@ -228,3 +228,13 @@
 
 # Relative filename of the reference count data file.
 refcount_file = 'data/refcounts.dat'
+
+# Sphinx 2 and Sphinx 3 compatibility
+# -----------------------------------
+
+# bpo-40204: Allow Sphinx 2 syntax in the C domain
+c_allow_pre_v3 = True
+
+# bpo-40204: Disable warnings on Sphinx 2 syntax of the C domain since the
+# documentation is built with -W (warnings treated as errors).
+c_warn_on_allowed_pre_v3 = False

Misc/NEWS.d/next/Documentation/2020-08-12-18-35-40.bpo-40204.C8A_pe.rst

@@ -0,0 +1,3 @@
+Enable Sphinx 3.2 ``c_allow_pre_v3`` option and disable
+``c_warn_on_allowed_pre_v3`` option to make the documentation compatible
+with Sphinx 2 and Sphinx 3.