Bump ecdsa from 0.13 to 0.13.3

Bumps [ecdsa](https://github.com/warner/python-ecdsa) from 0.13 to 0.13.3.
- [Release notes](https://github.com/warner/python-ecdsa/releases)
- [Changelog](https://github.com/warner/python-ecdsa/blob/python-ecdsa-0.13.3/NEWS)
- [Commits](tlsfuzzer/python-ecdsa@python-ecdsa-0.13...python-ecdsa-0.13.3)

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

Author: dependabot[bot]

requirements.txt

@@ -1,3 +1,3 @@
 cryptography==2.6.1
-ecdsa==0.13
+ecdsa==0.13.3
 yapf==0.21.0