Trac #30975: Silence packaging 20.5 deprecation warnings

packaging 20.5 throws lots of deprecation warnings
{{{
/usr/lib/python3.9/site-packages/packaging/version.py:127:
DeprecationWarning: Creating a LegacyVersion has been deprecated and
will be removed in the next major release
}}}
when called from setuptools when building the module index (which
happens indirectly eg. when calling sage_getdoc). We silence these
warnings, since this is really out of sage's control (even if all of
sage's python packages were fixed, this could still be triggered by
packages installed with sage-pip, or by any other python package in the
case of distro packages)

URL: https://trac.sagemath.org/30975
Reported by: arojas
Ticket author(s): Antonio Rojas
Reviewer(s): Matthias Koeppe

build/pkgs/packaging/install-requires.txt

@@ -1 +1,2 @@
 packaging >=18.0
+# Trac #30975: packaging 20.5 is known to work but we have to silence "DeprecationWarning: Creating a LegacyVersion"

src/sage/all.py

@@ -107,6 +107,10 @@
 warnings.filterwarnings('ignore', category=DeprecationWarning,
     module='.*ast')
 
+# Ignore packaging 20.5 deprecation warnings
+warnings.filterwarnings('ignore', category=DeprecationWarning,
+    module='.*packaging')
+
 ################ end setup warnings ###############################