SG-37774 Remove Python 2 traces

[carlos-villavicencio-adsk]

What's new?

• Removes Python 2 specific logic
• Removes dependency on SIX module
• Removes warnings related to SHOTGUN_PYTHON_VERSION env var
• Updates software_credits

Logic that will be kept

• _out_of_date_check logic introduced on SG-31059 Prevent Desktopstartup Auto Update When Running On Python2 #105 that will force the auto-update when running older versions of Python
• Warnings introduced on SG-20242: old site config warning #64 for tk-desktop versions older than v2.5 (~5 years ago).
• Warnings introduced on SG-17545 Adds a new error to catch using an old core with Python 3. #59 for tk-core versions older than v0.19 which doesn't support Python 3 (~5 years ago).