Fix attempt to unlink directory copied into source package (Python 3.9) (#3235)

Summary:
# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [x] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [N/A] Did you make sure to update the docs?
- [N/A] Did you write any new necessary tests?

## What does this PR do?

Currently when installing the newest source package from PyPI I get an error like so:

```
Collecting fairseq
  Using cached fairseq-0.10.2.tar.gz (938 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  ERROR: Command errored out with exit status 1:
   command: /home/frankier/sources/datasets/.venv/bin/python3 /tmp/tmp_ujftsgi_in_process.py get_requires_for_build_wheel /tmp/tmpmn0eumq2
       cwd: /tmp/pip-install-dg5d6q9y/fairseq
  Complete output (31 lines):
  Traceback (most recent call last):
    File "setup.py", line 214, in <module>
      do_setup(package_data)
    File "setup.py", line 136, in do_setup
      setup(
    File "/tmp/pip-build-env-hag0sxvp/overlay/lib/python3.9/site-packages/setuptools/__init__.py", line 152, in setup
      _install_setup_requires(attrs)
    File "/tmp/pip-build-env-hag0sxvp/overlay/lib/python3.9/site-packages/setuptools/__init__.py", line 147, in _install_setup_requires
      dist.fetch_build_eggs(dist.setup_requires)
    File "/tmp/pip-build-env-hag0sxvp/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 60, in fetch_build_eggs
      raise SetupRequirementsError(specifier_list)
  setuptools.build_meta.SetupRequirementsError: ['cython', 'numpy', 'setuptools>=18.0']

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "/tmp/tmp_ujftsgi_in_process.py", line 280, in <module>
      main()
    File "/tmp/tmp_ujftsgi_in_process.py", line 263, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/tmp/tmp_ujftsgi_in_process.py", line 114, in get_requires_for_build_wheel
      return hook(config_settings)
    File "/tmp/pip-build-env-hag0sxvp/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 149, in get_requires_for_build_wheel
      return self._get_build_requires(
    File "/tmp/pip-build-env-hag0sxvp/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 130, in _get_build_requires
      self.run_setup()
    File "/tmp/pip-build-env-hag0sxvp/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 145, in run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 217, in <module>
      os.unlink(fairseq_examples)
  IsADirectoryError: [Errno 21] Is a directory: 'fairseq/examples'
  ----------------------------------------
ERROR: Command errored out with exit status 1: /home/frankier/sources/datasets/.venv/bin/python3 /tmp/tmp_ujftsgi_in_process.py get_requires_for_build_wheel /tmp/tmpmn0eumq2 Check the logs for full command output.
```

I believe the reason for this is that the source package contains the examples directory because it was put there during package creation (it seems the symlink because a directory). Now, when setup.py is run again, it seems the setup.py attempts to unlink the directory, which is not possible because only symlinks can be unlinked. This PR therefore only attempts to unlink it if it is a symlink. I have not thoroughly tested whether my proposed cause is the true cause, but this should fix it in any case.

Note that the source package is fetched because there is no wheel for Python 3.9, so most users will not see this because they will use the wheel.

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �

Pull Request resolved: https://github.com/pytorch/fairseq/pull/3235

Reviewed By: alexeib

Differential Revision: D26513259

Pulled By: myleott

fbshipit-source-id: 775d6c636a5867b9983bb6419829f13ee414e2fd

setup.py

@@ -256,5 +256,5 @@ if __name__ == "__main__":
         }
         do_setup(package_data)
     finally:
-        if "build_ext" not in sys.argv[1:] and os.path.exists(fairseq_examples):
+        if "build_ext" not in sys.argv[1:] and os.path.islink(fairseq_examples):
             os.unlink(fairseq_examples)