The version in -proposed installed without the syntax warning:

$ sudo apt install python3-genshi=0.7.3-0ubuntu3.1
[sudo] password for sarnold: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  libfwupdplugin1
Use 'sudo apt autoremove' to remove it.
Suggested packages:
  doc-base python-genshi-doc
The following packages will be upgraded:
  python3-genshi
1 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.
Need to get 121 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://192.168.0.27/ubuntu focal-proposed/main amd64 python3-genshi all 0.7.3-0ubuntu3.1 [121 kB]
Fetched 121 kB in 0s (701 kB/s)        
(Reading database ... 209443 files and directories currently installed.)
Preparing to unpack .../python3-genshi_0.7.3-0ubuntu3.1_all.deb ...
Unpacking python3-genshi (0.7.3-0ubuntu3.1) over (0.7.3-0ubuntu3) ...
Setting up python3-genshi (0.7.3-0ubuntu3.1) ...


The tests supplied with the package work:

[sarnold:/usr/lib/python3/dist-packages/genshi/tests] $ for f in * ; do python3 $f ; done
........
----------------------------------------------------------------------
Ran 8 tests in 0.006s

OK
................................................/usr/lib/python3/dist-packages/genshi/template/interpolation.py:34: DeprecationWarning: Flags not at the start of the expression '[uU]?[rR]?("""|\\\'\\\'\\' (truncated)
  token_re = re.compile('%s|%s(?s)' % (
....
----------------------------------------------------------------------
Ran 52 tests in 0.022s

OK
...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................<string>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
<string>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
.<string>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
<string>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
...............................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 867 tests in 0.704s

OK
..................................
----------------------------------------------------------------------
Ran 34 tests in 0.014s

OK
....................................................
----------------------------------------------------------------------
Ran 52 tests in 0.019s

OK
.................................................................
----------------------------------------------------------------------
Ran 65 tests in 0.053s

OK
/usr/bin/python3: can't find '__main__' module in '__pycache__'
.......
----------------------------------------------------------------------
Ran 7 tests in 0.002s

OK


Alas testing genshi in context is very difficult for me at the moment; given the very small, targeted, change, this feels safe.

Thanks