Add defgroup for customize and suppressing byte-compile warnings

[syohex]

There are some warnings about this.

pytest.el:70:1:Warning: defcustom for `pytest-project-names' fails to specify                                                                                  
    containing group                                                                                                                                           
pytest.el:70:1:Warning: defcustom for `pytest-project-names' fails to specify                                                                                  
    containing group                                                                                                                                           
pytest.el:73:1:Warning: defcustom for `pytest-project-root-files' fails to                                                                                     
    specify containing group                                                                                                                                   
pytest.el:73:1:Warning: defcustom for `pytest-project-root-files' fails to                                                                                     
    specify containing group                                                                                                                                   
pytest.el:76:1:Warning: defcustom for `pytest-project-root-test' fails to                                                                                      
    specify containing group                                                                                                                                   
pytest.el:76:1:Warning: defcustom for `pytest-project-root-test' fails to                                                                                      
    specify containing group                                                                                                                                   
pytest.el:79:1:Warning: defcustom for `pytest-global-name' fails to specify                                                                                    
    containing group                                                                                                                                           
pytest.el:79:1:Warning: defcustom for `pytest-global-name' fails to specify                                                                                    
    containing group                                                                                                                                           
pytest.el:83:1:Warning: defcustom for `pytest-cmd-flags' fails to specify                                                                                      
    containing group                                                                                                                                           
pytest.el:83:1:Warning: defcustom for `pytest-cmd-flags' fails to specify                                                                                      
    containing group  

[ionrock]

Thanks @syohex!