Append '-editable' to version for editable installs

This will append '-editable' to the usb.__version__ string
when pyusb is installed via `python setup.py develop` or
`pip install -e .`

setup.py

@@ -29,7 +29,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-
+import sys
 from setuptools import setup
 from distutils.version import LooseVersion
 from setuptools import __version__ as setuptools_version
@@ -38,12 +38,29 @@
 if LooseVersion(setuptools_version).version[0] < 12:
     setuptools_scm += '<2.0'
 
-setup(
-    name='pyusb',
-    use_scm_version={
+
+def pyusb_scm_version():
+    """append '-editable' to version for editable installs"""
+    from setuptools_scm.version import get_local_node_and_date
+
+    def editable_local_scheme(version):
+        local_scheme = get_local_node_and_date(version)
+        # distutils scans sys.argv for matching command classes
+        # (see `distutils.dist.Distribution._parse_command_opts`)
+        if "develop" in sys.argv[1:]:
+            return local_scheme + "-editable"
+        return local_scheme
+
+    return {
         "version_scheme": "post-release",
+        "local_scheme": editable_local_scheme,
         "write_to": "usb/_version.py",
-    },
+    }
+
+
+setup(
+    name='pyusb',
+    use_scm_version=pyusb_scm_version,
     setup_requires=setuptools_scm,
     description='Python USB access module',
     author='Robert Wlodarczyk',