requirements-dev: pin matplotlib so the types gate can resolve it

Refs #9. scripts/plots/ccusage_*.py imports matplotlib, but nothing in
CI's types workflow installed it, so pyright reported 8
reportMissingImports errors there (locally it resolves via the system
python's 3.10.8, which is why the miss did not show up here). matplotlib
is an analyst-side plotting dependency — backend code never imports it —
so it belongs in the dev/toolchain pins, not backend/requirements.txt.

Co-Authored-By: Kimi K3 <[email protected]>

Author: Nitjsefnie

requirements-dev.txt

@@ -8,3 +8,8 @@ isort==8.0.1
 pycodestyle==2.14.0
 pylint==4.0.6
 pyright==1.1.408
+# Not a tool: the types gate installs this file so pyright can resolve
+# third-party imports, and scripts/plots/ccusage_*.py imports matplotlib.
+# It is an analyst-side plotting dependency, not a backend runtime dep,
+# so it does not belong in backend/requirements.txt.
+matplotlib==3.10.8