python/setup.py: incremental build fails due to already existing setup.py link

[jhgit]

Trying to build the python binding fails after the first attempt because it tries to create a link to setup.py that was created already in the first build attempt:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "setup.py", line 45, in <module>
    os.link('python/setup.py', 'setup.py')
FileExistsError: [Errno 17] File exists: 'python/setup.py' -> 'setup.py'

The following patch avoids creating the link if it already exists:

--- python/setup.py.orig    2023-01-08 10:03:32 UTC
+++ python/setup.py
@@ -41,7 +41,7 @@ template = 'python/MANIFEST.in'
 # distutils assume setup.py is in the root of the project
 # we need to include C source from the parent so trick it
 in_ucl_root = 'setup.py' in os.listdir('python')
-if in_ucl_root:
+if not os.path.isfile('setup.py') and in_ucl_root:
     os.link('python/setup.py', 'setup.py')

 setup(