Compatibility fixes for Blender 2.78c

Blender 2.78c is shipped with a version of the io_blend_utils module that
doesn't have a `pythonpath()` function yet, and that's bundled with an
older version of BAM. To work around this, we ship BAM as wheel, and detect
whether this version is needed to run.

As an added bonus, Blender 2.78c can now also use the file exclude filter
for Flamenco. The `bam_supports_exclude_option()` function is thus no
longer necessary.

blender_cloud/wheels/__init__.py

@@ -44,6 +44,12 @@ def load_wheel(module_name, fname_prefix):
                   module_name, module.__file__, fname_prefix)
         return
 
+    sys.path.append(wheel_filename(fname_prefix))
+    module = __import__(module_name)
+    log.debug('Loaded %s from %s', module_name, module.__file__)
+
+
+def wheel_filename(fname_prefix: str) -> str:
     path_pattern = os.path.join(my_dir, '%s*.whl' % fname_prefix)
     wheels = glob.glob(path_pattern)
     if not wheels:
@@ -51,9 +57,7 @@ def load_wheel(module_name, fname_prefix):
 
     # If there are multiple wheels that match, load the latest one.
     wheels.sort()
-    sys.path.append(wheels[-1])
-    module = __import__(module_name)
-    log.debug('Loaded %s from %s', module_name, module.__file__)
+    return wheels[-1]
 
 
 def load_wheels():