Update python-dotenv requirement from ~=0.20.0 to ~=0.21.0

Updates the requirements on [python-dotenv](https://github.com/theskumar/python-dotenv) to permit the latest version.
- [Release notes](https://github.com/theskumar/python-dotenv/releases)
- [Changelog](https://github.com/theskumar/python-dotenv/blob/main/CHANGELOG.md)
- [Commits](https://github.com/theskumar/python-dotenv/compare/v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: python-dotenv
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,5 +1,5 @@
 pyTelegramBotAPI~=4.5.1
-python-dotenv~=0.20.0
+python-dotenv~=0.21.0
 APScheduler~=3.9.1
 SQLAlchemy~=1.4.37
 pymysql==1.0.2