py-nbval: Add Python 38

[essandess]

py-nbval: Add Python 38

• Fixes: https://trac.macports.org/ticket/59952

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]

Travis Build #9965 Errored.

Lint results

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

Port py37-nbval's dependencies fail on xcode10.3. Log
Port py-nbval success on xcode10.3. Log
Port py37-nbval success on xcode9.4. Log
Port py-nbval success on xcode9.4. Log
Port py37-nbval success on xcode8.3. Log
Port py-nbval success on xcode8.3. Log
Port py37-nbval success on xcode7.3. Log
Port py-nbval success on xcode7.3. Log

The build timed out.