setup.py: test for FileExistsError on symlink

Multiple runs of setuptools encounter a FileExistsError exception
trying to re-symlink the same files.

This exception is safe to ignore: the files were already symlinked
so the call can be considered successful.

Signed-off-by: Sirio Balmelli <sirio@b-ad.ch>

release/setup.py

@@ -1,5 +1,8 @@
 from setuptools import setup
 import os
 for f in ['git-filter-repo', 'git_filter_repo.py', 'README.md']:
-  os.symlink("../"+f, f)
+    try:
+        os.symlink("../"+f, f)
+    except FileExistsError:
+        pass
 setup(use_scm_version=dict(root="..", relative_to=__file__))