Remove stray egg-info directory automatically.

If a user or contributor ran `pip install -e .` on transformers < 3.0,
pip created a transformers.egg-info directory next to the transformers
directory at the root of the repository.

In transformers 3.0, the source is in a `src` subdirectory.
`pip install -e .` creates a transformers.egg-info directory there.
However, pip will still pick transformers.egg-info from the previous
location. This is a bug: https://github.com/pypa/pip/issues/5466

Users and contributors are likely to hit this problem because the
documentation for transformers 3.0 relies heavily on extra_requires
which didn't exist in earlier versions, so aren't defined in a stale
transformers.egg-info directory.

If such a directory exists, remove it. It's autogenerated, gitignored
and not supposed to contain anything of value.

setup.py

@@ -34,9 +34,28 @@ To create the package for pypi.
 
 """
 
+import shutil
+from pathlib import Path
+
 from setuptools import find_packages, setup
 
 
+# Remove stale transformers.egg-info directory to avoid https://github.com/pypa/pip/issues/5466
+stale_egg_info = Path(__file__).parent / "transformers.egg-info"
+if stale_egg_info.exists():
+    print(
+        (
+            "Warning: {} exists.\n\n"
+            "If you recently updated transformers to 3.0 or later, this is expected,\n"
+            "but it may prevent transformers from installing in editable mode.\n\n"
+            "This directory is automatically generated by Python's packaging tools.\n"
+            "I will remove it now.\n\n"
+            "See https://github.com/pypa/pip/issues/5466 for details.\n"
+        ).format(stale_egg_info)
+    )
+    shutil.rmtree(stale_egg_info)
+
+
 extras = {}
 
 extras["mecab"] = ["mecab-python3"]