[Bug] gunittest: r3.flow test fails with Python 3.11

[nilason]

Describe the bug
The raster3d/r3.flow/testsuite/r3flow_test.py fails running on Python 3.11 :

ERROR: test_flowlines (__main__.FlowlineTest.test_flowlines)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "raster3d/r3.flow/testsuite/r3flow_test.py", line 140, in test_flowlines
    self.assertVectorAsciiEqualsVectorAscii(
  File "etc/python/grass/gunittest/case.py", line 1243, in assertVectorAsciiEqualsVectorAscii
    fromlines = open(actual, "U").readlines()
                ^^^^^^^^^^^^^^^^^
ValueError: invalid mode: 'U'

The 'U' mode has long been deprecated, Python 3.7 docs:

There is an additional mode character permitted, 'U', which no longer has any effect, and is considered deprecated. It previously enabled universal newlines in text mode, which became the default behaviour in Python 3.0. Refer to the documentation of the newline parameter for further details.

To Reproduce
Steps to reproduce the behavior:

1. Install Python 3.11
2. Run python raster3d/r3.flow/testsuite/r3flow_test.py

Expected behavior
The test should not fail because of this.

Screenshots
If applicable, add screenshots to help explain your problem.

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: all

GRASS 8.3.dev (2022)
PROJ: 9.1.0
GDAL/OGR: 3.6.0
GEOS: 3.11.1
SQLite: 3.40.0
Python: 3.11.0 | packaged by conda-forge | (main, Oct 25 2022, 06:24:51) [Clang 14.0.4 ]

Additional context
Test report is available as artifact here https://github.com/OSGeo/grass/suites/9568241766/artifacts/455907193