with block: frame.f_local is read-only

[ep12]

Describe the bug
Currently only global variables can be assigned new values. (status quo: see comments below)

To Reproduce

from pyopm.core import ObjectPattern
def myfunc():
    x = 5
    p = ObjectPattern({'obj': {'bind': {'x': None}}})
    m = p.match(dict())
    with m:
        print(x)

myfunc()

will print 5, because the locals cannot be updated (__enter__ -> _start_block).

Expected behavior
myfunc() should result in {} being printed.

Screenshots
None

Additional context
python 3.8.2

[ep12]

9cb5311 partially resolves this issue:

• locals can now be created and updated from pyopm.core._start_block and pyopm.core._end_block
• locals can not be deleted with cpython. It seems to work when using pypy.
  A fix for this would be a nice-to-have, however the main functionality to bind the matched values seems to work. A source of inspiration might be pdb, because pdb can delete locals using both cpython and pypy (but how?).

[ep12]

2b2f142 adds support for variables that are used with *_DEREF and *_CLOSURE access modes.
More testing is needed: Maybe the implementation is not stable, maybe one or two branches are unnecessary (this and this branch?)