tests: make Python-based servers compatible with Python 2 and 3

Update smbserver.py and negtelnetserver.py to be compatible with
Python 3 while staying backwards-compatible to support Python 2.

Fix string encoding and handling of echoed and transferred data.

Tested with both Python 2.7.17 and Python 3.7.7

Reported-by: Daniel Stenberg
Assisted-by: Kamil Dudka
Reviewed-by: Marcel Raad

Fixes #5104
Closes #5110

tests/curl_test_data.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
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