Fix for Python 3.10, 4 and 10

[hugovk]

It's generally safer to compare to the sys.version_info tuple than the sys.version string, which can include assumptions that each version component is always a single character.

---

There's also some code which uses six.PY3, similar to:

if six.PY3:
    print("Python 3+ code")
else:
    print "Python 2 code" 

Where:

PY3 = sys.version_info[0] == 3

When run on Python 4, this will run the Python 2 code!

Instead, use six.PY2.

---

Found with https://github.com/asottile/flake8-2020.

[coveralls]

Coverage decreased (-0.03%) to 96.964% when pulling 0cbfad8 on hugovk:fix-flake8-2020 into 7a24229 on warner:master.

[hugovk]

The Python 3.4 failure is unrelated and also happens on master, because it's EOL and dependencies are dropping support for it:

• https://travis-ci.org/hugovk/python-ecdsa/builds/637634120

[tomato42]

The Python 3.4 failure is unrelated and also happens on master, because it's EOL and dependencies are dropping support for it:

* https://travis-ci.org/hugovk/python-ecdsa/builds/637634120

we are targeting all python versions that have support, be it from community in Debian or from Red Hat in RHEL, we don't limit ourselves to upstream python support

[hugovk]

we are targeting all python versions that have support, be it from community in Debian or from Red Hat in RHEL, we don't limit ourselves to upstream python support

Sure, I guess you'll need to pin some more dependencies.

[tomato42]

ok, builds should be fixed with #183, could you rebase on current master?

[hugovk]

Done and Travis is ✅!

[tomato42]

looks good, thanks!

[hugovk]

You're welcome, thank you for maintaining ecdsa!