Retire the last references to the retired lecture-python repo (#968)

Nothing in this repo points at QuantEcon/lecture-python any more.

pandas_panel read realwage.csv and countries.csv from that repo while reading
employ.csv — the third file in the same trio, in the same lecture — from this
repo's own copy. All three copies are already committed under
_static/lecture_specific/pandas_panel/, so two of the three reads were
fetching from a repo last touched in January 2022 for no reason. Now all three
are consistent and use the same URL form.

troubleshooting pointed readers at that same dead repo's issue tracker to
report problems with this one. Repointed at this repo's issues, matching what
the sibling lecture repos do.

Verified byte-identical before repointing: each own-repo copy has the same
sha256 as the legacy file it replaces, and the new URLs serve those same
bytes, so lecture output cannot change.

Note this is deliberately an interim step. P2 of the datasets pilot migrates
this trio to QuantEcon/data-lectures and will rewrite these three lines again
— but it rewrites them whether they say "legacy" or "own-repo" today, so this
costs P2 nothing and stops the lecture depending on an unmaintained repo in
the meantime.

Part of QuantEcon/meta#337
See QuantEcon/meta#336

Co-authored-by: Claude Fable 5 <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://raw.githubusercontent.com/QuantEcon/lecture-python/master/source/_static/lecture_specific/pandas_panel/realwage.csv'
+url1 = 'https://github.com/QuantEcon/lecture-python.myst/raw/refs/heads/main/lectures/_static/lecture_specific/pandas_panel/realwage.csv'
 ```
 
 ```{code-cell} ipython3
@@ -185,7 +185,7 @@ function.
 The dataset can be accessed with the following link:
 
 ```{code-cell} ipython3
-url2 = 'https://raw.githubusercontent.com/QuantEcon/lecture-python/master/source/_static/lecture_specific/pandas_panel/countries.csv'
+url2 = 'https://github.com/QuantEcon/lecture-python.myst/raw/refs/heads/main/lectures/_static/lecture_specific/pandas_panel/countries.csv'
 ```
 
 ```{code-cell} ipython3

lectures/troubleshooting.md

@@ -64,7 +64,7 @@ touch.
 
 ## Reporting an Issue
 
-One way to give feedback is to raise an issue through our [issue tracker](https://github.com/QuantEcon/lecture-python/issues).
+One way to give feedback is to raise an issue through our [issue tracker](https://github.com/QuantEcon/lecture-python.myst/issues).
 
 Please be as specific as possible.  Tell us where the problem is and as much
 detail about your local set up as you can provide.