Unable to install anything using pip after upgrade to 20.0

**Environment**



* pip version: 20.0

* Python version: 3.6.8

* OS: Ubuntu 14.04.6 (Don't judge)



**Description**



During our build process, we automatically upgrade to the newest version on pip. After upgrading to 20.0 (using pip install -U pip setuptools), we are unable to install any packages.



**Output**



```

Traceback (most recent call last):

  File "/home/svc-web/webshop/env/bin/pip", line 8, in <module>

    sys.exit(main())

  File "/home/svc-web/webshop/env/lib/python3.6/site-packages/pip/_internal/cli/main.py", line 73, in main

    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))

  File "/home/svc-web/webshop/env/lib/python3.6/site-packages/pip/_internal/commands/__init__.py", line 96, in create_command

    module = importlib.import_module(module_path)

  File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module

    return _bootstrap._gcd_import(name[level:], package, level)

  File "<frozen importlib._bootstrap>", line 994, in _gcd_import

  File "<frozen importlib._bootstrap>", line 971, in _find_and_load

  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked

  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked

  File "<frozen importlib._bootstrap_external>", line 678, in exec_module

  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed

  File "/home/svc-web/webshop/env/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 24, in <module>

    from pip._internal.cli.req_command import RequirementCommand

  File "/home/svc-web/webshop/env/lib/python3.6/site-packages/pip/_internal/cli/req_command.py", line 20, in <module>

    from pip._internal.operations.prepare import RequirementPreparer

  File "/home/svc-web/webshop/env/lib/python3.6/site-packages/pip/_internal/operations/prepare.py", line 16, in <module>

    from pip._internal.distributions import (

  File "/home/svc-web/webshop/env/lib/python3.6/site-packages/pip/_internal/distributions/__init__.py", line 1, in <module>

    from pip._internal.distributions.source import SourceDistribution

ImportError: cannot import name 'SourceDistribution'

```

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