fix(serializer): Make sentry_repr dunder method to avoid mock problems

[sl0thentr0py]

The addition of sentry_repr in #1322 breaks MagicMock since it creates attrs on the fly.
This converts it to a dunder magic method __sentry_repr__.

See here that mock doesn't mess with custom magics and the following test code:

In [22]: from unittest import mock

In [23]: m = mock.Mock()

In [24]: m
Out[24]: <Mock id='140712474488560'>

In [25]: type(m)
Out[25]: unittest.mock.Mock

In [26]: getattr(m, "foo")
Out[26]: <Mock name='mock.foo' id='140712490825808'>

In [27]: getattr(m, "__foo__")
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Input In [27], in <module>
----> 1 getattr(m, "__foo__")

File /usr/lib/python3.10/unittest/mock.py:636, in NonCallableMock.__getattr__(self, name)
    634         raise AttributeError("Mock object has no attribute %r" % name)
    635 elif _is_magic(name):
--> 636     raise AttributeError(name)
    637 if not self._mock_unsafe:
    638     if name.startswith(('assert', 'assret', 'asert', 'aseert', 'assrt')):

AttributeError: __foo__