⬆️ Bump anaconda from 2026.06 to 2026.07 (#582)

Rolling monthly bump of the conda env pin. Opened by hand rather than by
Dependabot: this repo's weekly Dependabot slot had not yet fired for the
2026.07 release, and the rest of the Python lecture family has already
been validated and merged.

Validated the same way as the rest of the family — by dispatching
`cache.yml` on this branch for a full fresh `jb build lectures -W
--keep-going` under the new env. See QuantEcon/workspace-lectures#21 for
the cross-repo status.

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

Author: mmcky

environment.yml

@@ -3,7 +3,7 @@ channels:
   - default
 dependencies:
   - python=3.13
-  - anaconda=2026.06
+  - anaconda=2026.07
   - pip
   - pip:
     - jupyter-book>=1.0.4post1,<2.0