Implement PEP 489 multi-phase initialization for C extension

This commit modernizes the C extension to use PEP 489 multi-phase
initialization, enabling proper subinterpreter support and module
isolation for Python 3.12+.

Key changes:

Module State Management:
- Added maxminddb_state struct to store per-module state
- Implemented get_maxminddb_state() helpers to access module state
- Added module lifecycle functions (traverse, clear, free)

Type Conversion:
- Converted Reader_Type, Metadata_Type, and ReaderIter_Type from
  static types to heap types using PyType_FromModuleAndSpec()
- Created PyType_Spec definitions for all three types
- Removed static global type declarations

State Threading:
- Updated all functions to access module state instead of globals:
  - Reader_init(), get_record(), Reader_metadata()
  - Reader_iter(), ReaderIter_next()
  - from_entry_data_list(), from_map(), from_array()
- Removed static global variables for MaxMindDB_error and
  ipaddress_ip_network

Module Initialization:
- Implemented maxminddb_exec() to initialize module state
- Added module slots declaring:
  - Multi-phase initialization support (Py_mod_exec)
  - Multiple interpreter support (Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED)
  - GIL-free operation when available (Py_MOD_GIL_NOT_USED)
- Updated PyModuleDef with m_size, m_slots, and GC functions
- Simplified PyInit_extension() to use PyModuleDef_Init()

Testing:
- Updated test regex to handle fully qualified type names in error
  messages (heap types include module path)
- All 278 tests pass

Benefits:
- Supports Python 3.12+ isolated subinterpreters
- Enables multiple independent module instances
- Complements existing free-threading support (PEP 703)
- Follows modern Python C API best practices
- Future-proof for Python 3.14's InterpreterPoolExecutor

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: oschwald

HISTORY.rst

@@ -3,7 +3,7 @@
 History
 -------
 
-2.9.0
+3.0.0
 ++++++++++++++++++
 
 * IMPORTANT: Python 3.10 or greater is required. If you are using an older
@@ -15,6 +15,13 @@ History
   thread-safe for concurrent reads on platforms with pthread support (such as
   Linux and macOS) and Windows. On other platforms, the extension will use
   GIL-based protection.
+* The C extension now uses PEP 489 multi-phase initialization, enabling
+  proper subinterpreter support and module isolation for Python 3.12+. This
+  modernizes the extension to use heap types instead of static types and
+  implements per-module state management. Key benefits include support for
+  Python 3.12+ isolated subinterpreters, multiple independent module
+  instances, and future-proofing for Python 3.14's InterpreterPoolExecutor.
+  Requested by R. Christian McDonald in GitHub #105.
 
 2.8.2 (2025-07-25)
 ++++++++++++++++++