Add example unit test and config for testing

Add testing configuration:
 - pytest as test framework
 - tox for creating testing environments and running tests (run with `tox`)
 - pytest-asycio for asyncio testing
 - coverage/pytest-cov for measuring test coverage
 - flake8 for pep8 checking

I've also added one quick example test demonstrating basic unit testing
and use of the basic fixtures and marks provided by pytest-asyncio.

Just this already covers a suprising 32% of the codebase, mostly through
imports, but I wouldn't expect it to be helpful yet. This should provide
a good base to build on in the future though.

tests/telethon/test_helpers.py

@@ -0,0 +1,59 @@
+"""
+tests for telethon.helpers
+"""
+
+from base64 import b64decode
+
+import pytest
+
+from telethon import helpers
+
+
+def test_strip_text():
+    assert helpers.strip_text(" text ", []) == "text"
+    # I can't interpret the rest of the code well enough yet
+
+
+class TestSyncifyAsyncContext:
+    class NoopContextManager:
+        def __init__(self, loop):
+            self.count = 0
+            self.loop = loop
+
+        async def __aenter__(self):
+            self.count += 1
+            return self
+
+        async def __aexit__(self, exc_type, *args):
+            assert exc_type is None
+            self.count -= 1
+
+        __enter__ = helpers._sync_enter
+        __exit__ = helpers._sync_exit
+
+    def test_sync_acontext(self, event_loop):
+        contm = self.NoopContextManager(event_loop)
+        assert contm.count == 0
+
+        with contm:
+            assert contm.count == 1
+
+        assert contm.count == 0
+
+    @pytest.mark.asyncio
+    async def test_async_acontext(self, event_loop):
+        contm = self.NoopContextManager(event_loop)
+        assert contm.count == 0
+
+        async with contm:
+            assert contm.count == 1
+
+        assert contm.count == 0
+
+
+def test_generate_key_data_from_nonce():
+    gkdfn = helpers.generate_key_data_from_nonce
+
+    key_expect = b64decode(b'NFwRFB8Knw/kAmvPWjtrQauWysHClVfQh0UOAaABqZA=')
+    nonce_expect = b64decode(b'1AgjhU9eDvJRjFik73bjR2zZEATzL/jLu9yodYfWEgA=')
+    assert gkdfn(123456789, 1234567) == (key_expect, nonce_expect)