[python] drop nose usage for unittest

Description

nose component[1] is not maintainer, last commit is 7 years old(!).
It somehow managed to work so far(?) it is time to retire.
For a simple python language replacing the nose with pytest in generated test-requirements.txt and tox.ini working without additional change.

[1] https://github.com/nose-devs/nose

Swagger-codegen version

2.4.28

Swagger declaration file content or url

petstore

Command line used for generation

java -jar swagger-codegen-cli.jar generate -l python -i swagger.json -o out1

Steps to reproduce

$ python3 -V
Python 3.10.4
$ tox -e py3
<snip>
  File "/out1/.tox/py3/lib/python3.10/site-packages/nose/suite.py", line 106, in _set_tests
    if isinstance(tests, collections.Callable) and not is_suite:
AttributeError: module 'collections' has no attribute 'Callable'

Suggest a fix/enhancement

Apply:

diff --git a/test-requirements.txt b/test-requirements.txt
index 2702246..0ebd529 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -1,5 +1,5 @@
 coverage>=4.0.3
-nose>=1.3.7
 pluggy>=0.3.1
 py>=1.4.31
+pytest
 randomize>=0.13
diff --git a/tox.ini b/tox.ini
index 3d0be61..2d62656 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,5 +6,5 @@ deps=-r{toxinidir}/requirements.txt
      -r{toxinidir}/test-requirements.txt
 
 commands=
-   nosetests \
+   pytest \
       []

$ tox -e py3
<snip>
  py3: commands succeeded
  congratulations :)

In the generated Python output, inspect test-requirements.txt and tox.ini, starting with their test dependency and command entries shown in the issue. Run tox -e py3 against the generated petstore client and confirm the test command succeeds without the reported nose error.