properly write version file even if in pretend mode

[anarcat]

before this, SETUPTOOLS_SCM_PRETEND_VERSION make setuptools_scm fail
to write the write_to file correctly. this was apparently to avoid
issues with archives, which we believe is not a problem anymore.

this is, however, a problem for other tools like Debian packaging:
there, we use the pretend mechanism to import the version number from
the Debian packaging system (from debian/changelog, usually). this
works at setup time: setuptools_scm has the proper environment and
detects the version. but if the software was written to depend on the
write_to file, it will fail to find that version at runtime because
setup.py will not have written the write_to file properly at build
time.

this patch simply makes sure that the version is written even if the
provided version number is a string, which is usually the case for
pretend version numbers. a fixed test case is also provided.

Closes: #101

[anarcat]

test before patch:

[1039]anarcat@angela:setuptools_scm130$ py.test testing/test_functions.py 
=================================================== test session starts ====================================================
platform linux2 -- Python 2.7.9 -- py-1.4.25 -- pytest-2.6.3
setuptools version 28.0.0
setuptools_scm version 1.11.1
collected 15 items 

testing/test_functions.py ..............F

========================================================= FAILURES =========================================================
___________________________________________ test_dump_version_works_with_pretend ___________________________________________

tmpdir = local('/tmp/pytest-10/test_dump_version_works_with_p0')
monkeypatch = <_pytest.monkeypatch.monkeypatch instance at 0x7f6ff3a77908>

    def test_dump_version_works_with_pretend(tmpdir, monkeypatch):
        monkeypatch.setenv(PRETEND_KEY, '1.0')
        get_version(write_to=str(tmpdir.join('VERSION.txt')))
>       assert tmpdir.join('VERSION.txt').read() == '1.0'

testing/test_functions.py:64: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/lib/python2.7/dist-packages/py/_path/common.py:131: in read
    with self.open(mode) as f:
/usr/lib/python2.7/dist-packages/py/_path/local.py:344: in open
    return py.error.checked_call(open, self.strpath, mode)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <py._error.ErrorMaker object at 0x7f6ff3d3e650>, func = <built-in function open>
args = ('/tmp/pytest-10/test_dump_version_works_with_p0/VERSION.txt', 'r'), kwargs = {}, __tracebackhide__ = False
cls = <class 'py.error.ENOENT'>, value = IOError(2, 'No such file or directory'), tb = <traceback object at 0x7f6ff3a77ab8>
errno = 2

    def checked_call(self, func, *args, **kwargs):
        """ call a function and raise an errno-exception if applicable. """
        __tracebackhide__ = True
        try:
            return func(*args, **kwargs)
        except self.Error:
            raise
        except (OSError, EnvironmentError):
            cls, value, tb = sys.exc_info()
            if not hasattr(value, 'errno'):
                raise
            __tracebackhide__ = False
            errno = value.errno
            try:
                if not isinstance(value, WindowsError):
                    raise NameError
            except NameError:
                # we are not on Windows, or we got a proper OSError
                cls = self._geterrnoclass(errno)
            else:
                try:
                    cls = self._geterrnoclass(_winerrnomap[errno])
                except KeyError:
                    raise value
>           raise cls("%s%r" % (func.__name__, args))
E           ENOENT: [No such file or directory]: open('/tmp/pytest-10/test_dump_version_works_with_p0/VERSION.txt', 'r')

/usr/lib/python2.7/dist-packages/py/_error.py:84: ENOENT
--------------------------------------------------- Captured stdout call ---------------------------------------------------
root '/home/anarcat/src/setuptools_scm'
=========================================== 1 failed, 14 passed in 0.26 seconds ============================================

test after patch:

[1040]anarcat@angela:setuptools_scm1$ PYTHONPATH=. py.test testing/test_functions.py 
=================================================== test session starts ====================================================
platform linux2 -- Python 2.7.9 -- py-1.4.25 -- pytest-2.6.3
setuptools version 28.0.0
setuptools_scm version 1.11.1
collected 15 items 

testing/test_functions.py ...............

================================================ 15 passed in 0.21 seconds =================================================

[RonnyPfannschmidt]

well done