[rosdep] add python3 keys for pymodbus and urllib3 (#26056)

* add python3-pymodbus

debian: https://packages.debian.org/buster/python3-pymodbus
fedora: https://fedora.pkgs.org/32/fedora-x86_64/python3-pymodbus-2.3.0-2.fc32.noarch.rpm.html
ubuntu: https://packages.ubuntu.com/focal/python3-pymodbus

Signed-off-by: Mikael Arguedas <mikael.arguedas@gmail.com>

* add python3-urllib3

debian: https://packages.debian.org/buster/python3-urllib3
fedora: https://fedora.pkgs.org/32/fedora-x86_64/python3-urllib3-1.25.7-3.fc32.noarch.rpm.html
gentoo: https://packages.gentoo.org/packages/dev-python/urllib3
ubuntu: https://packages.ubuntu.com/focal/python3-urllib3

Signed-off-by: Mikael Arguedas <mikael.arguedas@gmail.com>

* add null rules for pymobdub on xenial and stretch

Signed-off-by: Mikael Arguedas <mikael.arguedas@gmail.com>

rosdep/python.yaml

@@ -6050,6 +6050,14 @@ python3-pykdl:
     '*': [python3-pykdl]
     'bionic': null
     'xenial': null
+python3-pymodbus:
+  debian:
+    '*': [python3-pymodbus]
+    stretch: null
+  fedora: [python3-pymodbus]
+  ubuntu:
+    '*': [python3-pymodbus]
+    xenial: null
 python3-pyparsing:
   arch: [python-pyparsing]
   debian: [python3-pyparsing]
@@ -6466,6 +6474,11 @@ python3-twisted:
     '*': ['python%{python3_pkgversion}-twisted']
     '7': null
   ubuntu: [python3-twisted]
+python3-urllib3:
+  debian: [python3-urllib3]
+  fedora: [python3-urllib3]
+  gentoo: [dev-python/urllib3]
+  ubuntu: [python3-urllib3]
 python3-usb:
   debian: [python3-usb]
   gentoo: [dev-python/pyusb]