specify version if none found

This is needed for the test script, where setuptools_scm doesn't work

gwinc/__init__.py

@@ -9,7 +9,7 @@ try:
     from ._version import __version__
 except ModuleNotFoundError:
     import setuptools_scm
-    __version__ = setuptools_scm.get_version()
+    __version__ = setuptools_scm.get_version(fallback_version='?.?.?')
 from .ifo import IFOS
 from .struct import Struct
 from .plot import plot_budget