Bump ccxt from 1.50.48 to 1.50.70

Bumps [ccxt](https://github.com/ccxt/ccxt) from 1.50.48 to 1.50.70.
- [Release notes](https://github.com/ccxt/ccxt/releases)
- [Changelog](https://github.com/ccxt/ccxt/blob/master/doc/exchanges-by-country.rst)
- [Commits](https://github.com/ccxt/ccxt/compare/1.50.48...1.50.70)

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

requirements.txt

@@ -1,7 +1,7 @@
 numpy==1.20.3
 pandas==1.2.4
 
-ccxt==1.50.48
+ccxt==1.50.70
 # Pin cryptography for now due to rust build errors with piwheels
 cryptography==3.4.7
 aiohttp==3.7.4.post0