Fix python3 relative import issue and setup.py

This change builds the protobuf files inside the osi3 package path
for Python2 to ensure that relative imports work, while not breaking
the general cmake build setup.  It also moves the protobuf generation
to the build command phase for tidiness.

Author: pmai

setup.py

@@ -4,10 +4,11 @@
 import os
 import subprocess
 import sys
+import re
 from distutils.spawn import find_executable
 
 from setuptools import setup
-from setuptools.command.install import install
+from setuptools.command.build_py import build_py
 
 # configure the version number
 from shutil import copyfile
@@ -24,7 +25,7 @@
 package_name = 'osi3'
 package_path = os.path.join(os.getcwd(), package_name)
 
-class GenerateProtobuf(install):
+class GenerateProtobufCommand(build_py):
 
     @staticmethod
     def find_protoc():
@@ -71,14 +72,23 @@ def find_protoc():
     """ Generate Protobuf Messages """
 
     def run(self):
+        if sys.hexversion >= 0x3000000:
+            pattern = re.compile('^import "osi_')
+            for source in self.osi_files:
+                with open(source) as src_file:
+                    with open(os.path.join(package_path, source),"w") as dst_file:
+                        for line in src_file:
+                            dst_file.write(pattern.sub('import "osi3/osi_',line))
         for source in self.osi_files:
             sys.stdout.write('Protobuf-compiling ' + source + '\n')
+            source_path = os.path.join(package_path, source) if sys.hexversion >= 0x3000000 else './' + source
             subprocess.check_call([self.find_protoc(),
+                                   '--proto_path=' + package_path,
                                    '--proto_path=.',
                                    '--python_out=' + package_path,
-                                   './' + source])
+                                   source_path])
 
-        install.run(self)
+        build_py.run(self)
 
 try:
     os.mkdir(package_path)
@@ -103,7 +113,7 @@ def run(self):
     packages=[package_name],
     install_requires=['protobuf'],
     cmdclass={
-        'install': GenerateProtobuf,
+        'build_py': GenerateProtobufCommand,
     },
     url='https://github.com/OpenSimulationInterface/open-simulation-interface',
     license="MPL 2.0",