Bump ccxt from 1.55.28 to 1.55.56

Bumps [ccxt](https://github.com/ccxt/ccxt) from 1.55.28 to 1.55.56.
- [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/1.55.28...1.55.56)

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

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

requirements.txt

@@ -1,7 +1,7 @@
 numpy==1.21.2
 pandas==1.3.2
 
-ccxt==1.55.28
+ccxt==1.55.56
 # Pin cryptography for now due to rust build errors with piwheels
 cryptography==3.4.7
 aiohttp==3.7.4.post0