c2chapel dependency changes (#26095)

[reviewed by @mppf]

Undo version upgrade for pycparser because it is not actually compatible
with the newest version of pycparserext (despite it apparently
installing cleanly if you didn't have an older version of pycparserext
installed on your machine, so far as I can tell)

Explicitly set the version of pycparserext to the latest version. This
should help avoid the testing mishap in the future, since explicitly
specifying the version makes it tell us if we upgrade to a version of
pycparser that is incompatible with it.

Update the Makefile for c2chapel to stay in line with the pycparser
version

Passed a full paratest with futures after having built c2chapel (in a
setting where I verified I was getting the failure)

third-party/chpl-venv/c2chapel-requirements.txt

@@ -1,3 +1,3 @@
 # tools/c2chapel/Makefile should match so fakeHeaders download matches
-pycparser==2.22
-pycparserext
+pycparser==2.20
+pycparserext==2021.1

tools/c2chapel/Makefile

@@ -35,7 +35,7 @@ link=$(bdir)/c2chapel
 
 # Note, this version is used only for the fake headers,
 # but it should probably match third-party/chpl-venv/c2chapel-requirements.txt
-VERSION=2.22
+VERSION=2.20
 TAR=release_v$(VERSION).tar.gz
 
 RELEASE=https://github.com/eliben/pycparser/archive/$(TAR)