Fix case of missing setuptools_scm

Author: avalentino

setup.cfg

@@ -46,6 +46,7 @@ doctest_optionflags =
 text_file_format = rst
 addopts = --doctest-rst
 xfail_strict = true
+norecursedirs = 'erfa/_dev'
 
 [flake8]
 max-line-length = 100

setup.py

@@ -148,9 +148,13 @@ def get_extensions():
     scm_version = types.ModuleType('scm_version')
     scm_version.__file__ = 'erfa/_dev/scm_version.py'
     code = compile(source, scm_version.__file__, 'exec')
-    exec(code, scm_version.__dict__)
-    guess_next_dev = functools.partial(scm_version._guess_next_dev,
-                                       liberfadir=LIBERFADIR)
+    try:
+        exec(code, scm_version.__dict__)
+    except ImportError:
+        guess_next_dev = None
+    else:
+        guess_next_dev = functools.partial(scm_version._guess_next_dev,
+                                           liberfadir=LIBERFADIR)
 
 use_scm_version = {
     'write_to': os.path.join('erfa', '_version.py'),