py-pip: add select groups for pip2 and pip3

[danchr]

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.x
Xcode 8.x

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?

• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[macportsbot]

Notifying maintainers:
@stromnov for port pip_select, py-pip.

[macportsbot]

Travis Build #6247 Passed.

Lint results

--->  Verifying Portfile for pip_select
--->  0 errors and 0 warnings found.
--->  Verifying Portfile for py-pip
--->  0 errors and 0 warnings found.

Port pip_select success on xcode10.2. Log
Port py27-pip success on xcode10.2. Log
Port py-pip success on xcode10.2. Log
Port py26-pip success on xcode10.2. Log
Port py33-pip success on xcode10.2. Log
Port py34-pip success on xcode10.2. Log
Port py35-pip success on xcode10.2. Log
Port py36-pip success on xcode10.2. Log
Port py37-pip success on xcode10.2. Log
Port pip_select success on xcode9.4. Log
Port py27-pip success on xcode9.4. Log
Port py-pip success on xcode9.4. Log
Port py26-pip success on xcode9.4. Log
Port py33-pip success on xcode9.4. Log
Port py34-pip success on xcode9.4. Log
Port py35-pip success on xcode9.4. Log
Port py36-pip success on xcode9.4. Log
Port py37-pip success on xcode9.4. Log
Port pip_select success on xcode8.3. Log
Port py27-pip success on xcode8.3. Log
Port py-pip success on xcode8.3. Log
Port py26-pip success on xcode8.3. Log
Port py33-pip success on xcode8.3. Log
Port py34-pip success on xcode8.3. Log
Port py35-pip success on xcode8.3. Log
Port py36-pip success on xcode8.3. Log
Port py37-pip success on xcode8.3. Log
Port pip_select success on xcode7.3. Log
Port py27-pip success on xcode7.3. Log
Port py-pip success on xcode7.3. Log
Port py26-pip success on xcode7.3. Log
Port py33-pip success on xcode7.3. Log
Port py34-pip success on xcode7.3. Log
Port py35-pip success on xcode7.3. Log
Port py36-pip success on xcode7.3. Log
Port py37-pip success on xcode7.3. Log

[cjones051073]

LGTM, but lets give @stromnov a few days to comment.

[reneeotten]

this is related to Trac 55977 and this PR should probably close that ticket.

[cjones051073]

Ok, long enough i think...