do not hardcode python executable name and use sys.executable

[kloczek]

Python executabe name should not be hardcoded.
Use instead sys.executable().

Without that fix pytest is failing in my build env where I have only python3.

+ /usr/bin/pytest -ra -m 'not network' --exit-0-if-no-units============================= test session starts ==============================platform linux -- Python 3.8.18, pytest-7.4.3, pluggy-1.3.0rootdir: /home/tkloczko/rpmbuild/BUILD/python-process-tests-3.0.0configfile: pytest.initestpaths: testscollected 3 itemstests/test_process_tests.py FFF [100%]=================================== FAILURES ===================================____________________________ test_wait_for_strings _____________________________tests/test_process_tests.py:16: in test_wait_for_strings with TestProcess('python', '-c', 'print("foobar")') as proc:../../BUILDROOT/python-process-tests-3.0.0-2.fc35.x86_64/usr/lib/python3.8/site-packages/process_tests.py:129: in __init__ self.proc = subprocess.Popen(args, stdout=stdout, **kwargs)/usr/lib64/python3.8/subprocess.py:858: in __init__ self._execute_child(args, executable, preexec_fn, close_fds,/usr/lib64/python3.8/subprocess.py:1720: in _execute_child raise child_exception_type(errno_num, err_msg, err_filename)E FileNotFoundError: [Errno 2] No such file or directory: 'python'_______________________________ test_filebuffer ________________________________tests/test_process_tests.py:25: in test_filebuffer with TestProcess('python', '-c', 'print("foobar")', stdout=fh) as proc:../../BUILDROOT/python-process-tests-3.0.0-2.fc35.x86_64/usr/lib/python3.8/site-packages/process_tests.py:133: in __init__ self.proc = subprocess.Popen(args, stdout=stdout, **kwargs)/usr/lib64/python3.8/subprocess.py:858: in __init__ self._execute_child(args, executable, preexec_fn, close_fds,/usr/lib64/python3.8/subprocess.py:1720: in _execute_child raise child_exception_type(errno_num, err_msg, err_filename)E FileNotFoundError: [Errno 2] No such file or directory: 'python'_________________________________ test_socket __________________________________tests/test_process_tests.py:33: in test_socket with TestProcess('python', '-mhttp.server', '0') as proc:../../BUILDROOT/python-process-tests-3.0.0-2.fc35.x86_64/usr/lib/python3.8/site-packages/process_tests.py:129: in __init__ self.proc = subprocess.Popen(args, stdout=stdout, **kwargs)/usr/lib64/python3.8/subprocess.py:858: in __init__ self._execute_child(args, executable, preexec_fn, close_fds,/usr/lib64/python3.8/subprocess.py:1720: in _execute_child raise child_exception_type(errno_num, err_msg, err_filename)E FileNotFoundError: [Errno 2] No such file or directory: 'python'=========================== short test summary info ============================FAILED tests/test_process_tests.py::test_wait_for_strings - FileNotFoundError...FAILED tests/test_process_tests.py::test_filebuffer - FileNotFoundError: [Err...FAILED tests/test_process_tests.py::test_socket - FileNotFoundError: [Errno 2...============================== 3 failed in 0.19s ===============================