Restore constraints pinning

I was under the mistaken impression that the current mpl releases were
compatible with pyparsing3, but looking at the linked mpl issue it's not
fixed yet only for their development so far. So to unblock CI this
restores the pinning as it's still needed until mpl releases a fix.

constraints.txt

@@ -1,3 +1,10 @@
 # jsonschema pinning needed due nbformat==5.1.3 using deprecated behaviour in
 # 4.0+. The pin can be removed after nbformat is updated.
 jsonschema==3.2.0
+
+# pyparsing restrictions are needed because pyparsing 3.0.0 and 3.0.1 break
+# matplotlib's mathtex extensions.  At the time of writing (2021-10-25), the
+# docs build is pinned to matplotlib<3.4 (current) because of deprecations, so
+# as we won't get matplotlib upgrades by default, this constraint likely can't
+# be removed until we can unpin matplotlib.
+pyparsing<3.0.0