Travis: test on both 5.2.x and 5.2.3-pending.

On those versions, pin check-manifest to 0.44 to prevent getting version conflicts for `build`, `pep517` and `virtualenv`.

We want to test with newer and older zope.interface versions, for #252
5.2.x is currently 5.2.2, with zope.interface 5.0.2.
5.2.3-pending has zope.interface 5.1.2.

This will fail once 5.2.3-pending is out of pending, but that is just a signal that I can clean this Travis config up again.

.travis.yml

@@ -22,6 +22,10 @@ jobs:
       env: PLONE_VERSION=5.2.x
       dist: bionic
       sudo: true
+    - python: "3.8"
+      env: PLONE_VERSION=5.2.3-pending
+      dist: bionic
+      sudo: true
 
 cache:
   pip: true

tests-5.2.3-pending.cfg

@@ -0,0 +1,40 @@
+[buildout]
+extends =
+    https://github.com/raw/collective/buildout.plonetest/master/test-5.2.x.cfg
+    https://dist.plone.org/release/5.2.3-pending/versions.cfg
+    https://github.com/raw/collective/buildout.plonetest/master/qa.cfg
+    https://github.com/raw/plone/plone.app.robotframework/master/versions.cfg
+    base.cfg
+
+parts +=
+    createcoverage
+
+package-name = collective.easyform
+package-extras = [test]
+test-eggs =
+
+# Python3 compatibility for plone.formwidget.z3cform is not released
+extensions +=
+    mr.developer
+sources = sources
+sources-dir = share
+auto-checkout +=
+    plone.formwidget.recaptcha
+
+[versions]
+setuptools =
+zc.buildout =
+coverage = >=3.7
+plone.app.mosaic =
+plone.app.robotframework = 1.5.0
+# check-manifest = 0.45 requires 'build' 0.1.0.
+# This requires a newer 'pep517' than is pinned by Plone,
+# and on Python 2.7 a newer virtualenv than is pinned by Plone.
+check-manifest = 0.44
+
+[sources]
+plone.formwidget.recaptcha = git ${remotes:plone}/plone.formwidget.recaptcha.git pushurl=${remotes:plone_push}/plone.formwidget.recaptcha.git
+
+[remotes]
+plone = https://github.com/plone
+plone_push = git@github.com:plone

tests-5.2.x.cfg

@@ -26,6 +26,10 @@ zc.buildout =
 coverage = >=3.7
 plone.app.mosaic =
 plone.app.robotframework = 1.5.0
+# check-manifest = 0.45 requires 'build' 0.1.0.
+# This requires a newer 'pep517' than is pinned by Plone,
+# and on Python 2.7 a newer virtualenv than is pinned by Plone.
+check-manifest = 0.44
 
 [sources]
 plone.formwidget.recaptcha = git ${remotes:plone}/plone.formwidget.recaptcha.git pushurl=${remotes:plone_push}/plone.formwidget.recaptcha.git