P2: repoint pandas_panel URLs to data-lectures (fix missing edit)

The prior commit deleted the local copies but did not stage the URL change in
pandas_panel.md, so the lecture still pointed at the now-deleted own-repo path.
Repoint url1/url2/url3 to the canonical data-lectures copies.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/pandas_panel.md

@@ -65,7 +65,7 @@ countries and assign it to `realwage`.
 The dataset can be accessed with the following link:
 
 ```{code-cell} ipython3
-url1 = 'https://github.com/QuantEcon/lecture-python.myst/raw/refs/heads/main/lectures/_static/lecture_specific/pandas_panel/realwage.csv'
+url1 = 'https://github.com/QuantEcon/data-lectures/raw/main/lectures/realwage.csv'
 ```
 
 ```{code-cell} ipython3
@@ -185,7 +185,7 @@ function.
 The dataset can be accessed with the following link:
 
 ```{code-cell} ipython3
-url2 = 'https://github.com/QuantEcon/lecture-python.myst/raw/refs/heads/main/lectures/_static/lecture_specific/pandas_panel/countries.csv'
+url2 = 'https://github.com/QuantEcon/data-lectures/raw/main/lectures/countries.csv'
 ```
 
 ```{code-cell} ipython3
@@ -502,7 +502,7 @@ in Europe by age and sex from [Eurostat](https://ec.europa.eu/eurostat/data/data
 The dataset can be accessed with the following link:
 
 ```{code-cell} ipython3
-url3 = 'https://github.com/QuantEcon/lecture-python.myst/raw/refs/heads/main/lectures/_static/lecture_specific/pandas_panel/employ.csv'
+url3 = 'https://github.com/QuantEcon/data-lectures/raw/main/lectures/employ.csv'
 ```
 
 Reading in the CSV file returns a panel dataset in long format. Use `.pivot_table()` to construct