Fix OSError when failing to create config directory on read-only file system

When calling the completion code, kitty tries to access the config directory and create it if it does not exist. If kitty has no permission to create it, a temporary directory will be created instead. This will fail on a read-only file system because that raises an `OSError` and not a `PermissionError`.
In practice this happens when building kitty on macOS Catalina using Nix because `HOME` is set to `/homeless-shelter` for purity and `/` is a read-only filesystem.

kitty/constants.py

@@ -5,6 +5,7 @@
 import os
 import pwd
 import sys
+import errno
 from collections import namedtuple
 from contextlib import suppress
 
@@ -57,13 +58,7 @@ def _get_config_dir():
             if os.access(q, os.W_OK) and os.path.exists(os.path.join(q, 'kitty.conf')):
                 return q
 
-    candidate = os.path.abspath(os.path.expanduser(os.environ.get('XDG_CONFIG_HOME') or '~/.config'))
-    ans = os.path.join(candidate, appname)
-    try:
-        os.makedirs(ans, exist_ok=True)
-    except FileExistsError:
-        raise SystemExit('A file {} already exists. It must be a directory, not a file.'.format(ans))
-    except PermissionError:
+    def make_tmp_conf():
         import tempfile
         import atexit
         ans = tempfile.mkdtemp(prefix='kitty-conf-')
@@ -73,6 +68,19 @@ def cleanup():
             with suppress(Exception):
                 shutil.rmtree(ans)
         atexit.register(cleanup)
+
+    candidate = os.path.abspath(os.path.expanduser(os.environ.get('XDG_CONFIG_HOME') or '~/.config'))
+    ans = os.path.join(candidate, appname)
+    try:
+        os.makedirs(ans, exist_ok=True)
+    except FileExistsError:
+        raise SystemExit('A file {} already exists. It must be a directory, not a file.'.format(ans))
+    except PermissionError:
+        make_tmp_conf()
+    except OSError as err:
+        if err.errno != errno.EROFS:  # Error other than read-only file system
+            raise
+        make_tmp_conf()
     return ans