Use raw string literals if backslashes involved

[maxnikulin]

Avoid ambiguity if backslashes start escape sequences.

[maxnikulin]

I was checking if pytest could provide more informative report for #34

python3 -m pytest test

...

=============================== warnings summary ===============================
test/test_server.py:218
  /home/ubuntu/danny0838/PyWebScrapBook/test/test_server.py:218: DeprecationWarning: invalid escape sequence \P
    f.write("""[server]

test/test_server.py:227
  /home/ubuntu/danny0838/PyWebScrapBook/test/test_server.py:227: DeprecationWarning: invalid escape sequence \P
    mock_browser.assert_called_once_with('"C:\Program Files\Mozilla Firefox\firefox.exe" %s &')

-- Docs: https://docs.pytest.org/en/stable/warnings.html
=========================== short test summary info ============================

[maxnikulin]

By the way, is there any protection against peculiar arguments for substitutions? On linux I strongly prefer something from execve(2) family that has each command line argument as a separate array element.

Path to firefox at least surrounded with quotes... Is it assumed that argument for the %s substitution is passed through urlencode, so it is safe?

[danny0838]

Implemented with slightly fix.