Handle missing extension correctly

If the extension module cannot be imported, the fallback path
would trigger ‘NameError: name 'maxminddb' is not defined’.

While at it:

- Use an underscored name for the optional extension import to hide it
  a bit from view (e.g. autocomplete in interactive Python shells)

- Use relative imports not unnecessarily repeat the module name
  everywhere

maxminddb/__init__.py

@@ -2,21 +2,23 @@
 import os
 from typing import IO, AnyStr, Union, cast
 
-try:
-    import maxminddb.extension
-except ImportError:
-    maxminddb.extension = None  # type: ignore
-
-from maxminddb.const import (
+from .const import (
     MODE_AUTO,
     MODE_FD,
     MODE_FILE,
     MODE_MEMORY,
     MODE_MMAP,
     MODE_MMAP_EXT,
 )
-from maxminddb.decoder import InvalidDatabaseError
-from maxminddb.reader import Reader
+from .decoder import InvalidDatabaseError
+from .reader import Reader
+
+try:
+    # pylint: disable=import-self
+    from . import extension as _extension
+except ImportError:
+    _extension = None  # type: ignore[assignment]
+
 
 __all__ = [
     "InvalidDatabaseError",
@@ -60,7 +62,7 @@ def open_database(
     ):
         raise ValueError(f"Unsupported open mode: {mode}")
 
-    has_extension = maxminddb.extension and hasattr(maxminddb.extension, "Reader")
+    has_extension = _extension and hasattr(_extension, "Reader")
     use_extension = has_extension if mode == MODE_AUTO else mode == MODE_MMAP_EXT
 
     if not use_extension:
@@ -75,7 +77,7 @@ def open_database(
     # checking purposes, pretend it is one. (Ideally this would be a subclass
     # of, or share a common parent class with, the Python Reader
     # implementation.)
-    return cast(Reader, maxminddb.extension.Reader(database, mode))
+    return cast(Reader, _extension.Reader(database, mode))
 
 
 __title__ = "maxminddb"

tests/reader_test.py

@@ -250,16 +250,16 @@ def test_opening_path(self):
             self.assertEqual(reader.metadata().database_type, "MaxMind DB Decoder Test")
 
     def test_no_extension_exception(self):
-        real_extension = maxminddb.extension
-        maxminddb.extension = None
+        real_extension = maxminddb._extension
+        maxminddb._extension = None
         with self.assertRaisesRegex(
             ValueError,
             "MODE_MMAP_EXT requires the maxminddb.extension module to be available",
         ):
             open_database(
                 "tests/data/test-data/MaxMind-DB-test-decoder.mmdb", MODE_MMAP_EXT
             )
-        maxminddb.extension = real_extension
+        maxminddb._extension = real_extension
 
     def test_broken_database(self):
         reader = open_database(