TypeError: required field "posonlyargs" missing from arguments

Summary

Installation on Ubuntu 20.04. Trying to access web interface results in 500 response in many situations.

Looks to be related to and explained by:

https://github.com/odoo/odoo/issues/40232

https://github.com/odoo/odoo/issues/40303

Steps to reproduce

Install on a distro that uses Python 3.8 and try to use admin interface

What is the current bug behavior?

A cascade of exceptions; 500 response code

What is the expected correct behavior?

No exceptions

Relevant logs and/or screenshots

2020-04-03 13:59:21,813 789370 ERROR elloe-flectra werkzeug: Error on request:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/flectra/tools/cache.py", line 88, in lookup
    r = d[key]
  File "/usr/lib/python3/dist-packages/flectra/tools/func.py", line 69, in wrapper
    return func(self, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/flectra/tools/lru.py", line 44, in __getitem__
    a = self.d[obj].me
KeyError: ('ir.qweb', <function IrQWeb.compile at 0x7f35627b8280>, 185, ('en_US', None, None, None, None, None))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/flectra/addons/base/ir/ir_qweb/qweb.py", line 329, in compile
    unsafe_eval(compile(astmod, '<template>', 'exec'), ns)
TypeError: required field "posonlyargs" missing from arguments

During handling of the above exception, another exception occurred:
...

Possible fixes

Introduce Python 3.8 compatibility.