WIP: Add __frozendict__ to type and modules

[vstinner]

Changes:

• Add _imp._module_set_frozendict() function.
• importlib calls _module_set_frozendict() if a module namespace contains frozendict (and is true).

Modify functions to handle/support frozendict:

• _PyDict_LookupIndexAndValue()
• _PyModule_Clear()
• module_dir()

Use __frozendict__ on stdlib modules:

• _abc
• _ast
• _operator
• _stat
• _struct
• _thread
• _types
• _warnings
• _weakref
• _weakrefset
• errno
• fcntl
• marshal
• math
• posix
• select
• signal
• stat
• textwrap
• token
• weakref

Stdlib modules which cannot use __frozendict__:

• linecache: test_linecache overrides a module attribute.
• keyword: test_keyword modifies module attributes.
• os: test_os overrides os atttributes.
• pwd: test_getpass modifies pwd atttributes.
• struct: test_struct modifies a struct attribute.
• time: time.tzset() modifies attributes such as time.timezone and time.daylight.
• site: test_site modify site attributes.
• sys: sys.ps1 can be modified by design.
• termios: test_pyrepl overrides termios attributes.
• traceback: test_traceback overrides multiple traceback attributes.
• _thread: test_idle overrides _thread.interrupt_main().
• warnings: warnings.showwarning() can be overriden by design.

Using __frozendict__ on Mapping and MutableMapping of collections.abc doesn't work because ABCMeta.__new__() calls _abc_init(cls) which sets attributes.

[vstinner]

TODO: 3 tests of test_typing are still failing.

[vstinner]

Reading __annotate__ or __annotations__ attribute of a type may have to modify the type dictionary. For now, I added a hack to not modify the dict if it's a frozendict. But maybe the type dict should be replaced with a new updated frozendict.