Ensure rx._callback resolves accessors

[philippjfr]

Previously accessors weren't resolved correctly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.70%. Comparing base (c0fa26e) to head (008394c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
+ Coverage   86.53%   86.70%   +0.16%     
==========================================
  Files          10       10              
  Lines        5148     5151       +3     
==========================================
+ Hits         4455     4466      +11     
+ Misses        693      685       -8     

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[maximlt]

Here's how to reproduce the reported issue. The output should display the accessor str method (e.g. <pandas.core.strings.accessor.StringMethods at 0x122c71ee0>), not the island Series. I added a simple unit test.

Code

import pandas as pd
import param

from param import rx

URL = 'https://datasets.holoviz.org/penguins/v1/penguins.csv'
df = rx(pd.read_csv(URL))

df['island'].str