Bump exchange-calendars from 4.7 to 4.8 in /etc/requirements_dependabot

Bumps [exchange-calendars](https://github.com/gerrymanoim/exchange_calendars) from 4.7 to 4.8.
- [Release notes](https://github.com/gerrymanoim/exchange_calendars/releases)
- [Changelog](https://github.com/gerrymanoim/exchange_calendars/blob/master/docs/changes_archive.md)
- [Commits](gerrymanoim/exchange_calendars@4.7...4.8)

---
updated-dependencies:
- dependency-name: exchange-calendars
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

etc/requirements_dependabot/requirements_tests.txt

@@ -27,7 +27,7 @@ exceptiongroup==1.2.2
     # via
     #   hypothesis
     #   pytest
-exchange-calendars==4.7
+exchange-calendars==4.8
     # via market-prices (pyproject.toml)
 flake8==7.1.1
     # via