Bump ccxt from 1.66.66 to 1.68.20

Bumps [ccxt](https://github.com/ccxt/ccxt) from 1.66.66 to 1.68.20.
- [Release notes](https://github.com/ccxt/ccxt/releases)
- [Changelog](https://github.com/ccxt/ccxt/blob/master/exchanges.cfg)
- [Commits](https://github.com/ccxt/ccxt/compare/1.66.66...1.68.20)

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

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

requirements.txt

@@ -3,7 +3,7 @@ numpy==1.22.0; python_version > '3.7'
 pandas==1.3.5
 pandas-ta==0.3.14b
 
-ccxt==1.66.66
+ccxt==1.68.20
 # Pin cryptography for now due to rust build errors with piwheels
 cryptography==36.0.1
 aiohttp==3.8.1