[Build][202012] Pin the toposort version to 1.7 in python2

[xumia]

Why I did it

Fix the docker-base-stretch build issue in nephos platform. see
https://dev.azure.com/mssonic/build/_build/results?buildId=223711&view=logs&j=1111c693-39b7-5a16-3aee-406c790b3886&t=c5703f10-3eba-5015-d19a-321c17c94619

Collecting supervisord-dependent-startup==1.4.0
  Downloading https://files.pythonhosted.org/packages/0e/66/d386c3d2cf36846ded0b9f4999a6c59fd04fb15d768e59d3acfa039102bc/supervisord_dependent_startup-1.4.0-py2.py3-none-any.whl
Collecting toposort>=1.5 (from supervisord-dependent-startup==1.4.0)
  Downloading https://files.pythonhosted.org/packages/8d/c3/44e51b42160145e4ebeb7d86ab93bd933fa9498f94e183ef8a47c6de8b2f/toposort-1.9.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    IOError: [Errno 2] No such file or directory: '/tmp/pip-build-LnROQE/toposort/setup.py'
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-LnROQE/toposort/

The other platforms have been upgraded to docker-base-buster, not impacted.

How I did it

Pin the toposort version to 1.7, the package supervisord-dependent-startup has dependency on it.
The toposort>=1.8 only for python3, is not applicable to python2.

How to verify it

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)