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.

setup.py

@@ -101,6 +101,11 @@ class BuildWheels(Command):
             log.info('Downloading Pillar Python SDK wheel')
             self.download_wheel(requirements['pillarsdk'])
 
+        # Download BAM from pypi. This is required for compatibility with Blender 2.78.
+        if not list(self.wheels_path.glob('blender_bam*.whl')):
+            log.info('Downloading BAM wheel')
+            self.download_wheel(requirements['blender-bam'])
+
         # Build CacheControl.
         if not list(self.wheels_path.glob('CacheControl*.whl')):
             log.info('Building CacheControl in %s', self.cachecontrol_path)