PyShp 2.1.1

Improvements:

• Handle shapes with no coords and represent as geojson with no coords (GeoJSON null-equivalent)
• Expand testing to Python 3.6, 3.7, 3.8 and PyPy; drop 3.3 and 3.4
• Added pytest testing

Bug fixes:

• Fix incorrect geo interface handling of multipolygons with complex exterior-hole relations
• Enforce shapefile requirement of at least one field, to avoid writing invalid shapefiles
• Fix Reader geo interface including DeletionFlag field in feature properties
• Fix polygons not being auto closed, which was accidentally dropped
• Fix error for null geometries in feature geojson
• Misc docstring cleanup