Reorganization of tank_vendor to support "vendor-less" shotgun-api3

[mathbou]

Hi there,

This PR is a requirement for this one SG-32061 Python3.7+ Syntax in the shotgun-api3 project.

A third_partyfolder is added in tank_vendor to act like a site-packages folder. The main purpose is to support a pip installation of the shotgun-api3 package and its sub-dependencies declared in its setup.py

[codecov]

Codecov Report

Merging #914 (6fc1311) into master (02b0538) will decrease coverage by 6.06%.
The diff coverage is 98.90%.

@@            Coverage Diff             @@
##           master     #914      +/-   ##
==========================================
- Coverage   78.91%   72.86%   -6.06%     
==========================================
  Files         182      182              
  Lines       20083    20079       -4     
==========================================
- Hits        15849    14631    -1218     
- Misses       4234     5448    +1214     

Files Changed	Coverage Δ
python/tank/platform/qt/tankqdialog.py	53.53% <ø> (-0.32%)	⬇️
python/tank/util/shotgun/download.py	79.41% <50.00%> (ø)
python/tank/api.py	88.79% <100.00%> (-0.44%)	⬇️
...thon/tank/authentication/console_authentication.py	92.70% <100.00%> (ø)
python/tank/authentication/login_dialog.py	91.22% <100.00%> (ø)
python/tank/authentication/session_cache.py	80.00% <100.00%> (ø)
python/tank/authentication/shotgun_wrapper.py	90.00% <100.00%> (ø)
python/tank/authentication/site_info.py	94.00% <100.00%> (ø)
python/tank/authentication/unified_login_flow2.py	86.52% <100.00%> (ø)
python/tank/authentication/user_impl.py	82.03% <100.00%> (-1.20%)	⬇️
... and 50 more

... and 24 files with indirect coverage changes

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