Add rosdep keys for pyparsing and pytest. (#16549)

* Add rosdep keys for pyparsing and pytest.

pytest and pyparsing are needed for ROS 2 development.

pytest is the new recommended test runner for ament_python packages (as far as I understand).

pyparsing is used in the current implementation of the conditional dependencies [[1]] for package.xml format 3

[1]: ros-infrastructure/rep#143

Package references:

* pyparsing
    * arch https://www.archlinux.org/packages/extra/any/python-pyparsing/
    * debian https://packages.debian.org/stable/python3-pyparsing
    * fedora https://apps.fedoraproject.org/packages/python3-pyparsing
    * gentoo https://packages.gentoo.org/packages/dev-python/pyparsing
    * ubuntu
        * trusty https://packages.ubuntu.com/trusty/python-pyparsing
        * xenial https://packages.ubuntu.com/xenial/python-pyparsing
        * artful https://packages.ubuntu.com/artful/python-pyparsing
        * bionic https://packages.ubuntu.com/bionic/python-pyparsing

* pytest
    * arch https://www.archlinux.org/packages/community/any/python-pytest/
    * debian https://packages.debian.org/stable/python3-pytest
    * fedora (2)  https://apps.fedoraproject.org/packages/python3-pytest/sources/
    * gentoo https://packages.gentoo.org/packages/dev-python/pytest
    * ubuntu
        * trusty https://packages.ubuntu.com/trusty/python-pytest
        * xenial https://packages.ubuntu.com/xenial/python-pytest
        * artful https://packages.ubuntu.com/artful/python-pytest
        * bionic https://packages.ubuntu.com/bionic/python-pytest

(2) Fedora's python3-pytest is an EPEL-only package.
I'm not sure if it should be included.

* Drop gentoo key.

I don't know whether gentoo has pip support via rosdep or if it's
possible to specfiy keys for masked packages.
For now I am just dropping it.

* Create a -pip key for python3-pytest on platforms where it is too old for ROS 2.

* Add python3-pytest key for Ubunbu Bionic.

Bionic is the first release that will include python3-pytest 3.2 for ROS 2.

* Remove python3-pytest-pip key.

* Don't bother scoping pytest to bionic.

* Re-add python3-pytest keys for debian and gentoo.

rosdep/python.yaml

@@ -4077,6 +4077,17 @@ python3-pep8:
 python3-pkg-resources:
   debian: [python3-pkg-resources]
   ubuntu: [python3-pkg-resources]
+python3-pyparsing:
+  arch: [python-pyparsing]
+  debian: [python3-pyparsing]
+  fedora: [python3-pyparsing]
+  gentoo: [dev-python/pyparsing]
+  ubuntu: [python3-pyparsing]
+python3-pytest:
+  arch: [python-pytest]
+  debian: [python3-pytest]
+  gentoo: [dev-python/pytest]
+  ubuntu: [python3-pytest]
 python3-ruamel.yaml:
   debian:
     buster: [python3-ruamel.yaml]