py-pyxb: update to 1.2.6, add Python 3.7 subport, whitespace changes

[chrstphrchvz]

Description

Note that PyXB is being discontinued by its current developer, and there doesn't yet appear to be new developer or fork interested in taking over the project. I have left a note linking to the relevant issue: pabigot/pyxb#100

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.14.2 18C54
Xcode 10.1 10B61
Python 3.7.2

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
  Kept whitespace changes in separate commit; can squash further if desired.
• 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]

Travis Build #4900 Passed.

Lint results

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

Port py27-pyxb success on xcode9.4. Log
Port py-pyxb success on xcode9.4. Log
Port py34-pyxb success on xcode9.4. Log
Port py35-pyxb success on xcode9.4. Log
Port py36-pyxb success on xcode9.4. Log
Port py37-pyxb success on xcode9.4. Log
Port py27-pyxb success on xcode8.3. Log
Port py-pyxb success on xcode8.3. Log
Port py34-pyxb success on xcode8.3. Log
Port py35-pyxb success on xcode8.3. Log
Port py36-pyxb success on xcode8.3. Log
Port py37-pyxb success on xcode8.3. Log
Port py27-pyxb success on xcode7.3. Log
Port py-pyxb success on xcode7.3. Log
Port py34-pyxb success on xcode7.3. Log
Port py35-pyxb success on xcode7.3. Log
Port py36-pyxb success on xcode7.3. Log
Port py37-pyxb success on xcode7.3. Log
Port py27-pyxb success on xcode10.1. Log
Port py-pyxb success on xcode10.1. Log
Port py34-pyxb success on xcode10.1. Log
Port py35-pyxb success on xcode10.1. Log
Port py36-pyxb success on xcode10.1. Log

[pmetzger]

Merged. Thanks, @chrstphrchvz!