Fix pyperformance venv (w/o any subcommand) exception

[maurycy]

Now:

2026-01-26T19:29:17.403748000+0100 maurycy@gimel /Users/maurycy/work/pyperformance (fix-428-venv) % uv run --python 3.14 --with . python -m pyperformance venv
usage: pyperformance [-h] [-V] {run,show,compare,list,list_groups,compile,compile_all,upload,venv} ...
Compares the performance of baseline_python with changed_python and prints a report.
positional arguments:
{run,show,compare,list,list_groups,compile,compile_all,upload,venv}
run Run benchmarks on the running python
show Display a benchmark file
compare Compare two benchmark files
list List benchmarks of the running Python
list_groups List benchmark groups of the running Python
compile Compile and install CPython and run benchmarks on installed Python
compile_all Compile and install CPython and run benchmarks on installed Python on all branches and revisions of CONFIG_FILE
upload Upload JSON results to a Codespeed website
venv Actions on the virtual environment
options:
-h, --help show this help message and exit
-V, --version show program's version number and exit

Before:

[1] 2026-03-23T10:53:38.530937000+0100 maurycy@gimel /Users/maurycy/src/github.com/maurycy/pyperformance (main) % uv run --python 3.14 --with . python -m pyperformance venv
Traceback (most recent call last):
File "<frozen runpy>", line 198, in _run_module_as_main
File "<frozen runpy>", line 88, in _run_code
File "/Users/maurycy/src/github.com/maurycy/pyperformance/pyperformance/__main__.py", line 3, in <module>
pyperformance.cli.main()
~~~~~~~~~~~~~~~~~~~~~~^^
File "/Users/maurycy/src/github.com/maurycy/pyperformance/pyperformance/cli.py", line 422, in main
_main()
~~~~~^^
File "/Users/maurycy/src/github.com/maurycy/pyperformance/pyperformance/cli.py", line 371, in _main
info = _pythoninfo.get_info(options.python)
^^^^^^^^^^^^^^
AttributeError: 'Namespace' object has no attribute 'python'

Fixes#428