Compat with pip10+

[kiorky]

This fixes #2001

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" git@github.com:corpusops/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[amancevice]

This might be a longer-term solution (in case pip changes again):

import pkg_resources

try:
    pkg_resources.get_distribution('docker-py')
    print(
        'ERROR: "docker-py" needs to be uninstalled before installing this'
        ' package:\npip uninstall docker-py', file=sys.stderr
    )
    sys.exit(1)
except pkg_resources.DistributionNotFound:
    pass

[shin-]

Thanks! I went ahead and removed the check altogether (accb9de) as I believe it is no longer useful.