Do not build unless setup.py is top-level scope (#2969)

I.e. put all file system altering operations under `if __name__ == "__main__":`

setup.py

@@ -44,7 +44,6 @@ if os.getenv('BUILD_VERSION'):
     version = os.getenv('BUILD_VERSION')
 elif sha != 'Unknown':
     version += '+' + sha[:7]
-print("Building wheel {}-{}".format(package_name, version))
 
 
 def write_version_file():
@@ -57,10 +56,6 @@ def write_version_file():
         f.write("    cuda = _check_cuda_version()\n")
 
 
-write_version_file()
-
-readme = open('README.rst').read()
-
 pytorch_dep = 'torch'
 if os.getenv('PYTORCH_VERSION'):
     pytorch_dep += "==" + os.getenv('PYTORCH_VERSION')
@@ -397,7 +392,15 @@ class clean(distutils.command.clean.clean):
         distutils.command.clean.clean.run(self)
 
 
-setup(
+if __name__ == "__main__":
+    print("Building wheel {}-{}".format(package_name, version))
+
+    write_version_file()
+
+    with open('README.rst') as f:
+        readme = f.read()
+
+    setup(
         # Metadata
         name=package_name,
         version=version,
@@ -423,4 +426,4 @@ setup(
             'build_ext': BuildExtension.with_options(no_python_abi_suffix=True),
             'clean': clean,
         }
-)
+    )