MAINT: drop pip installs that no lecture uses (#803)

* FIX: install quantecon in networks.md

`lectures/networks.md` imports `quantecon` but its pip cell never installs
it. The lecture builds today only because `markov_chains_I`, `markov_chains_II`
and `eigen_II` appear earlier in the TOC and install `quantecon` into the same
runner environment, so a full cold build satisfies the dependency by accident.

That breaks as soon as the lecture executes on its own — the normal case with a
warm `jupyter-cache`, where only the edited notebook is re-run. Observed in the
Chinese edition, which carries the same gap as a faithful translation.

Adds `quantecon` to the lecture's own pip cell and splits the cell to one
package per line, matching `input_output.md` and the style guide.

Closes #801

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

* MAINT: drop pip installs that no lecture uses

Three installed packages are never imported or used:

- `input_output.md` installs `quantecon` and `pandas-datareader`; the lecture
  only ever uses `quantecon_book_networks`, and has no `qe.` call anywhere.
- `networks.md` installs `pandas-datareader` but never touches it.

`pandas_datareader` is imported by exactly one lecture, `business_cycle.md`,
which installs it itself. `quantecon` is imported by `eigen_II`,
`markov_chains_I`, `markov_chains_II` and `networks`, each of which now
installs it itself. Neither is a declared dependency of
`quantecon-book-networks==1.6`, so nothing pulls them in transitively.

`xlrd` in `inflation_history.md` is deliberately left alone: it is never
imported but is required by `pd.read_excel` to read the lecture's `.xls`
dataset.

Builds on #802 — that PR makes `networks.md` install its own `quantecon`,
without which removing `input_output.md`'s copy would reopen the same hole
from the other end.

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

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mmcky <8263752+mmcky@users.noreply.github.com>

Author: 3 people

lectures/input_output.md

@@ -21,8 +21,6 @@ This lecture requires the following imports and installs before we proceed.
 :tags: [hide-output]
 
 !pip install quantecon_book_networks==1.6
-!pip install quantecon
-!pip install pandas-datareader
 ```
 
 ```{code-cell} ipython3

lectures/networks.md

@@ -18,7 +18,6 @@ kernelspec:
 
 !pip install quantecon
 !pip install quantecon-book-networks==1.6
-!pip install pandas-datareader
 ```
 
 ## Outline