requirements: bump attrs 21.2.0 -> 21.3.0

[Bastian-Krause]

Description
twisted 22.10.0 [1] (released on 2022-10-30) uses attrs imports such as..

from attrs import something

The new import ("attrs", not "attr") is supported since attrs 21.3.0 [2]. labgrid currently pins attrs to 21.2.0. This incompatibility causes issues such as..

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.11.0/x64/bin/crossbar", line 8, in <module>
    sys.exit(run())
             ^^^^^
  File "/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/crossbar/__init__.py", line 143, in run
    from autobahn.twisted import install_reactor
  File "/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/autobahn/twisted/__init__.py", line 40, in <module>
    from autobahn.twisted.websocket import \
  File "/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/autobahn/twisted/websocket.py", line 35, in <module>
    from twisted.internet import endpoints
  File "/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/twisted/internet/endpoints.py", line 63, in <module>
    from twisted.python.systemd import ListenFDs
  File "/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/twisted/python/systemd.py", line 18, in <module>
    from attrs import Factory, define
ModuleNotFoundError: No module named 'attrs'

While this should have been a dependency resolving issue (twisted requiring a newer version than labgrid pins), it manifests itself as the error above due to an incorrectly specified attrs dependency in twisted [3].

To fix this, bump attrs to 21.3.0.

[1] https://github.com/twisted/twisted/releases/tag/twisted-22.10.0
[2] https://github.com/python-attrs/attrs/blob/965972eba3db4ed1b7eca53fa19aed1f3fcd166b/CHANGELOG.rst#2130-2021-12-28
[3] twisted/twisted#11737

Checklist

• PR has been tested (via test-suite only)

[Bastian-Krause]

attrs 21.3.0 is breaking code coverage: python-attrs/attrs#895

We could either bump to attrs 21.4.0 or try to work around this by specifying coverage's source. I am not sure how the latter would work with pytest-cov, though.

[jluebbe]

It seems the simplest would be to move to a newer attrs, it that doesn't conflict with other dependencies.