Close NamedTemporaryFile early for Windows compat

[userzimmermann]

Hi. Running python manage.py assets build or watch causes the following error on Windows:

webassets.exceptions.FilterError: elm: subprocess returned a non-success result code: 1,
stdout=b'Success! Compiled 0 modules.\r\n',
stderr=b'elm-make: C:\\Users\\Zimmermann\\AppData\\Local\\Temp\\tmph2k6lcv_.js: openFile: permission denied (Permission denied)\n'

The problem comes from Python's tempfile.NamedTemporaryFile. As long as the file is not closed in Python, it can't be accessed by external processes on Windows. You can read about that here:

https://bugs.python.org/issue14243

and in the tempfile documentation:

https://docs.python.org/3/library/tempfile.html#tempfile.NamedTemporaryFile

The solution is just to create the temp file with detete=False, close it immediately, and delete it manually when it's not used anymore. webassets.filter.ExternalTool.subprocess itself uses a similar approach, only with tempfile.mkstemp instead.

[coveralls]

Coverage remained the same at 100.0% when pulling 27f9ea9 on userzimmermann:zimmermann/windows into 8ac1320 on cuducos:master.