Python3-compatible imports

pdbfixer/__init__.py

-from pdbfixer import PDBFixer
+from __future__ import absolute_import
+from .pdbfixer import PDBFixer
+

pdbfixer/pdbfixer.py

@@ -28,6 +28,7 @@ DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 USE OR OTHER DEALINGS IN THE SOFTWARE.
 """
+from __future__ import absolute_import
 __author__ = "Peter Eastman"
 __version__ = "1.1"
 
@@ -1027,8 +1028,7 @@ class PDBFixer(object):
 def main():
     if len(sys.argv) < 2:
         # Display the UI.
-        
-        import ui
+        from . import ui
         ui.launchUI()
     else:
         # Run in command line mode.

pdbfixer/ui.py

-import simtk.openmm.app as app
-from simtk.openmm.app.internal.pdbstructure import PdbStructure
-import simtk.unit as unit
-from pdbfixer import PDBFixer, substitutions, proteinResidues, dnaResidues, rnaResidues
-import uiserver
+from __future__ import absolute_import
+
 import webbrowser
 import os.path
-import gzip
 import time
-from io import BytesIO
+
+import simtk.openmm.app as app
+import simtk.unit as unit
+
+from .pdbfixer import PDBFixer, proteinResidues, dnaResidues, rnaResidues
+from . import uiserver
+
 try:
     from urllib.request import urlopen
     from io import StringIO