[fix]: make mongoshake-stat compatible with Python 2 and Python 3

[hyeonghwa]

Summary

• Replace urllib2 with try/except import for Python 2/3 compatibility
• Add from __future__ import print_function for print() compatibility
• Replace unicode type check with str
• Replace dict.has_key() with in operator
• Wrap dict.keys() with list() for sortability in Python 3

Test plan

• Run python2 ./mongoshake-stat --port=9100 and verify no SyntaxError
• Run python3 ./mongoshake-stat --port=9100 and verify no SyntaxError
• Verify output table formatting is intact on both versions

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