Handle mkdir error like Python 3.5

Apparently, EEXIST is not reliable, but if the directory exists that would definitely raise an OSError. That's the only acceptable error case, so allow only that. The current implementation would cause problems if the path exists and is a file.

This is copying the CPython implementation: https://github.com/python/cpython/blob/master/Lib/pathlib.py#L1219

nsist/pypi.py

@@ -111,9 +111,9 @@ class WheelDownloader(object):
         download_to = get_cache_dir() / 'pypi' / self.name / self.version
         try:
             download_to.mkdir(parents=True)
-        except OSError as e:
-            # Py2 compatible equivalent of FileExistsError
-            if e.errno != errno.EEXIST:
+        except OSError:
+            # Ignore OSError only if the directory exists
+            if not download_to.is_dir():
                 raise
         target = download_to / preferred_release.filename
 
@@ -188,8 +188,8 @@ def extract_wheel(whl_file, target_dir):
                 # shutil.copytree will not combine them.
                 try:
                     target.joinpath(p.name).mkdir()
-                except OSError as e:
-                    if e.errno != errno.EEXIST:
+                except OSError:
+                    if not target.joinpath(p.name).is_dir():
                         raise
                 merge_dir_to(p, target / p.name)
             else: