Fix tox configuration and replace scaffold placeholder test

[dholbach]

Remove the non-existent 'all' extras from tox.ini, delete the leftover test_skeleton.py (imports a module that never existed), and add a minimal smoke test so pytest has something to collect.

❯ tox -q
========================================================================== test session starts ==========================================================================
platform linux -- Python 3.14.6, pytest-9.1.1, pluggy-1.6.0 -- /var/home/daniel/dev/GLS_account_statement_reader/.tox/default/bin/python
cachedir: .tox/default/.pytest_cache
rootdir: /var/home/daniel/dev/GLS_account_statement_reader
configfile: setup.cfg
testpaths: tests
plugins: cov-7.1.0
collected 2 items                                                                                                                                                       

tests/test_imports.py::test_package_importable PASSED                                                                                                             [ 50%]
tests/test_imports.py::test_bookings_empty PASSED                                                                                                                 [100%]

============================================================================ tests coverage =============================================================================
____________________________________________________________ coverage: platform linux, python 3.14.6-final-0 ____________________________________________________________

Name                                                Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------------------
src/bank_statement_reader/__init__.py                  11      1      0      0    91%   13
src/bank_statement_reader/booking/__init__.py           5      0      0      0   100%
src/bank_statement_reader/booking/booking_base.py     164    124     76      0    17%   53-58, 62, 66-79, 83, 87-93, 97, 102, 106, 113-130, 134-137, 140-197, 201, 204-254, 258, 261-262, 282-291, 295-296
src/bank_statement_reader/bookings.py                  97     65     42      0    23%   23-31, 34, 44-45, 48-54, 58, 62, 66, 70, 73-75, 98-119, 122-150, 153-165, 168, 171
src/bank_statement_reader/cli.py                       19     19      2      0     0%   2-57
src/bank_statement_reader/exceptions.py                 4      0      0      0   100%
src/bank_statement_reader/statement_reader.py         152    126     54      0    13%   45-52, 59-99, 104-165, 181-196, 207-242, 246-253, 257-259, 263-275
-----------------------------------------------------------------------------------------------
TOTAL                                                 452    335    174      0    19%
=========================================================================== 2 passed in 0.29s ===========================================================================
  default: OK (3.81 seconds)
  congratulations :) (3.85 seconds)

[CarliJoy]

Nice idea. But as this project has no real tests or CI I rather would delete the tests and tox config.

Having import only tests are not very useful.

Feel free to contribute some real tests or delete them.