Only error if directory really does not exist

On Windows, a user is getting an error from makedirs() with the argument
of the current directory '.'.  Per the comments in a more recent `os.py`
let’s refuse to error out if the directory exists.

Hopefully, fix #283.

skyfield/iokit.py

@@ -97,7 +97,7 @@ def __init__(self, directory, verbose=True, expire=True):
         try:
             os.makedirs(self.directory)
         except OSError as e:
-            if e.errno != errno.EEXIST:
+            if e.errno != errno.EEXIST and not os.path.isdir(name):
                 raise
 
         # Each instance gets its own copy of these data structures,