python3-speedtest-cli: fix RDEPENDS

There are packages missing in RDEPENDS needed to run speedtest-cli. Add
them and use += for the assignment as we don't know what inherited
classes may have added.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-python/recipes-devtools/python/python3-speedtest-cli_2.1.3.bb

@@ -7,4 +7,8 @@ SRC_URI[sha256sum] = "5e2773233cedb5fa3d8120eb7f97bcc4974b5221b254d33ff16e2f1d41
 
 inherit pypi setuptools3
 
-RDEPENDS:${PN} = "${PYTHON_PN}-misc"
+RDEPENDS:${PN} += " \
+    python3-misc \
+    python3-threading \
+    python3-xml \
+"