fix: fix context shallow copy

[Kiprey]

Classic python object shallow-copy problem.

context = {
    'lastvar': last_var,
    'lines': [],
    'variables': {},
    'interesting_lines': [],
    'force_var_reuse': False
}
...
while len(context['lines']) < num_lines:
    tmp_context = context.copy()
    try:
        if (random.random() < self._interesting_line_prob) and (len(tmp_context['interesting_lines']) > 0):
            tmp_context['force_var_reuse'] = True
            lineno = random.choice(tmp_context['interesting_lines'])
        else:
            lineno = random.choice(self._all_nonhelper_lines)
        creator = self._creators['line'][lineno]
        self._expand_rule('line', creator, tmp_context, 0, False)
        context = tmp_context
    except RecursionError as e:
        print('Warning: ' + str(e))

When RecursionError is triggered, some of the generated lines are retained in the original context. This can lead to some unintended behavior, such as variable redefinition.