Fix broken raw GitHub URLs for CSV data files (#491)

The repo restructure (drop .myst suffix, master→main) broke two
raw.githubusercontent.com URLs used in code cells:

- pandas.md: test_pwt.csv → 404 (causes cache build failure)
- python_advanced_features.md: test_table.csv → 404

Updated both from the old path:
  master/source/_static/lecture_specific/...
to the current path:
  main/lectures/_static/lecture_specific/...

Author: mmcky

lectures/pandas.md

@@ -168,7 +168,7 @@ The dataset contains the following indicators
 We'll read this in from a URL using the `pandas` function `read_csv`.
 
 ```{code-cell} ipython3
-df = pd.read_csv('https://raw.githubusercontent.com/QuantEcon/lecture-python-programming/master/source/_static/lecture_specific/pandas/data/test_pwt.csv')
+df = pd.read_csv('https://raw.githubusercontent.com/QuantEcon/lecture-python-programming/main/lectures/_static/lecture_specific/pandas/data/test_pwt.csv')
 type(df)
 ```
 

lectures/python_advanced_features.md

@@ -1170,7 +1170,7 @@ In summary, iterables
 :label: paf_ex1
 ```
 
-Complete the following code, and test it using [this csv file](https://raw.githubusercontent.com/QuantEcon/lecture-python-programming/master/source/_static/lecture_specific/python_advanced_features/test_table.csv), which we assume that you've put in your current working directory
+Complete the following code, and test it using [this csv file](https://raw.githubusercontent.com/QuantEcon/lecture-python-programming/main/lectures/_static/lecture_specific/python_advanced_features/test_table.csv), which we assume that you've put in your current working directory
 
 ```{code-block} python3
 :class: no-execute