Update pymodbus requirement from ~=3.6.9 to ~=3.8.3

Updates the requirements on [pymodbus](https://github.com/pymodbus-dev/pymodbus) to permit the latest version.
- [Release notes](https://github.com/pymodbus-dev/pymodbus/releases)
- [Changelog](https://github.com/pymodbus-dev/pymodbus/blob/dev/CHANGELOG.rst)
- [Commits](pymodbus-dev/pymodbus@v3.6.9...v3.8.3)

---
updated-dependencies:
- dependency-name: pymodbus
  dependency-type: direct:production
...

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

custom_components/idm_heatpump/requirements.txt

@@ -1 +1 @@
-pymodbus~=3.6.9
+pymodbus~=3.8.3