title: ImportError: cannot import name 'SourceDistribution' from 'pip._internal.distributions.source'
body: <p><code>pip3 install</code> is not working and also pip3 is not being able to downgrade to pip19 from pip20.0:</p>
  
  <pre><code>Rayaans-MacBook-Pro:~ rayaangrewal$ pip3 install 
      Traceback (most recent call last):
        File "/Library/Frameworks/Python.framework/Versions/3.7/bin/pip3", line 10, in &lt;module&gt;
          sys.exit(main())
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/cli/main.py", line 73, in main
          command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/commands/__init__.py", line 96, in create_command
          module = importlib.import_module(module_path)
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
          return _bootstrap._gcd_import(name[level:], package, level)
        File "&lt;frozen importlib._bootstrap&gt;", line 1006, in _gcd_import
        File "&lt;frozen importlib._bootstrap&gt;", line 983, in _find_and_load
        File "&lt;frozen importlib._bootstrap&gt;", line 967, in _find_and_load_unlocked
        File "&lt;frozen importlib._bootstrap&gt;", line 677, in _load_unlocked
        File "&lt;frozen importlib._bootstrap_external&gt;", line 728, in exec_module
        File "&lt;frozen importlib._bootstrap&gt;", line 219, in _call_with_frames_removed
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/commands/install.py", line 24, in &lt;module&gt;
          from pip._internal.cli.req_command import RequirementCommand
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/cli/req_command.py", line 20, in &lt;module&gt;
          from pip._internal.operations.prepare import RequirementPreparer
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/operations/prepare.py", line 16, in &lt;module&gt;
          from pip._internal.distributions import (
        File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/distributions/__init__.py", line 1, in &lt;module&gt;
          from pip._internal.distributions.source import SourceDistribution
      ImportError: cannot import name 'SourceDistribution' from 'pip._internal.distributions.source' (/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/distributions/source/__init__.py)
  </code></pre>