Specifying explicit transitive dependency for uvloop due to Python version conflict (#3675)

Adding a temporary fix until uvicorn fix is released

Author: nicknezis

heron/tools/ui/src/python/BUILD

@@ -13,6 +13,7 @@ pex_library(
         "jinja2==2.11.2",
         "aiofiles==0.5.0",
         "uvicorn==0.11.7",
+        "uvloop==0.14.0",
     ],
     deps = [
         "//heron/common/src/python:common-py",