support/testing: fix the TestPythonPy[2,3]Incremental test cases

The recent python-incremental version bump (commit
fd03cb5a12981a9c34b0025ff3ae4b8456342724) added a change/fix
in incremental.Version().

python-incremental 17.5.0 return: "[package, version 1.2.3rc4]")
python-incremental 21.3.0 return: "[package, version 1.2.3.rc4]")

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1710552692
https://gitlab.com/buildroot.org/buildroot/-/jobs/1710552693

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/package/sample_python_incremental.py

@@ -1,3 +1,3 @@
 import incremental
 v = incremental.Version("package", 1, 2, 3, release_candidate=4)
-assert(str(v) == "[package, version 1.2.3rc4]")
+assert(str(v) == "[package, version 1.2.3.rc4]")