Pin quantecon_book_networks==1.6 to unblock input_output + networks

quantecon_book_networks 1.6 migrates off the removed `pkg_resources`
module to the stdlib `importlib.resources`, so the package imports again
on the anaconda 2026.06 stack (and remains compatible with 2025.12).

This unblocks the input_output and networks lectures, whose execution
aborted with `ModuleNotFoundError: No module named 'pkg_resources'` under
the 2026.06 upgrade. Picks up the upstream fix from
QuantEcon/quantecon_book_networks#21.

Addresses cause 2 of #775.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: mmcky

lectures/input_output.md

@@ -20,7 +20,7 @@ This lecture requires the following imports and installs before we proceed.
 ```{code-cell} ipython3
 :tags: [hide-output]
 
-!pip install quantecon_book_networks
+!pip install quantecon_book_networks==1.6
 !pip install quantecon
 !pip install pandas-datareader
 ```

lectures/networks.md

@@ -16,7 +16,7 @@ kernelspec:
 ```{code-cell} ipython3
 :tags: [hide-output]
 
-!pip install quantecon-book-networks pandas-datareader
+!pip install quantecon-book-networks==1.6 pandas-datareader
 ```
 
 ## Outline