## memory error on co_annotations

I modifled py_compile to add `from __future__ import co_annotations`
automatically.

```
$ ../co_annotations/python -m compileall mypy
Listing 'mypy'...
Compiling 'mypy/checker.py'...
free(): corrupted unsorted chunks
Aborted

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007ffff7c73859 in __GI_abort () at abort.c:79
#2  0x00007ffff7cde3ee in __libc_message
(action=action@entry=do_abort, fmt=fmt@entry=0x7ffff7e08285 "%s\n") at
../sysdeps/posix/libc_fatal.c:155
#3  0x00007ffff7ce647c in malloc_printerr
(str=str@entry=0x7ffff7e0a718 "free(): corrupted unsorted chunks") at
malloc.c:5347
#4  0x00007ffff7ce81c2 in _int_free (av=0x7ffff7e39b80 <main_arena>,
p=0x555555d1db30, have_lock=<optimized out>) at malloc.c:4356
#5  0x0000555555603906 in PyMem_RawFree (ptr=<optimized out>) at
Objects/obmalloc.c:1922
#6  _PyObject_Free (ctx=<optimized out>, p=<optimized out>) at
Objects/obmalloc.c:1922
#7  _PyObject_Free (ctx=<optimized out>, p=<optimized out>) at
Objects/obmalloc.c:1913
#8  0x000055555567caa9 in compiler_unit_free (u=0x555555ef0fd0) at
Python/compile.c:583
#9  0x000055555568aea5 in compiler_exit_scope (c=0x7fffffffc3d0) at
Python/compile.c:760
#10 compiler_function (c=0x7fffffffc3d0, s=<optimized out>,
is_async=0) at Python/compile.c:2529
#11 0x000055555568837d in compiler_visit_stmt (s=<optimized out>,
c=0x7fffffffc3d0) at Python/compile.c:3665
#12 compiler_body (c=c@entry=0x7fffffffc3d0, stmts=0x555556222450) at
Python/compile.c:1977
#13 0x0000555555688e51 in compiler_class (c=c@entry=0x7fffffffc3d0,
s=s@entry=0x555556222a60) at Python/compile.c:2623
#14 0x0000555555687ce3 in compiler_visit_stmt (s=<optimized out>,
c=0x7fffffffc3d0) at Python/compile.c:3667
#15 compiler_body (c=c@entry=0x7fffffffc3d0, stmts=0x5555563014c0) at
Python/compile.c:1977
#16 0x000055555568db00 in compiler_mod (filename=0x7ffff72e6770,
mod=0x5555563017b0, c=0x7fffffffc3d0) at Python/compile.c:2001
```