[WORKSPACE, bazel] Remove references to PY2 and funcsign

The funcsign module is no longer used by "tree".

Author: charlesbeattie

WORKSPACE

@@ -45,15 +45,6 @@ http_archive(
     urls = ["https://github.com/abseil/abseil-py/archive/main.zip"],
 )
 
-http_archive(
-    name = "funcsigs_archive",
-    build_file = "@//bazel:funcsigs.BUILD",
-    strip_prefix = "funcsigs-1.0.2",
-    urls = [
-        "https://pypi.python.org/packages/94/4a/db842e7a0545de1cdb0439bb80e6e42dfe82aaeaadd4072f2263a4fbed23/funcsigs-1.0.2.tar.gz",
-    ],
-)
-
 http_archive(
     name = "eigen_archive",
     build_file = "@//bazel:eigen.BUILD",

bazel/dm_env.BUILD

@@ -11,7 +11,6 @@ py_library(
         "dm_env/specs.py",
     ],
     visibility = ["//visibility:public"],
-    deps = ["@funcsigs_archive//:funcsigs"],
 )
 
 py_library(

bazel/funcsigs.BUILD

@@ -23,7 +23,7 @@ py_library(
     name = "tree",
     srcs = ["tree/__init__.py"],
     data = [":tree/_tree.so"],
-    srcs_version = "PY2AND3",
+    srcs_version = "PY3",
     visibility = ["@dm_env_archive//:__pkg__"],
     deps = [":sequence"],
 )