Switch to new http_archive rules so that building with bazel 0.20 works.

Bazel deprecated new_http_archive in version 0.20, which causes an error at
build time (unless a temporary flag is used to renable new_http_archive).

Resolves #86

PiperOrigin-RevId: 224024305

WORKSPACE

@@ -13,35 +13,37 @@
 # limitations under the License.
 workspace(name = "io_abseil_py")
 
-new_http_archive(
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
     name = "six_archive",
     urls = [
         "http://mirror.bazel.build/pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz",
         "https://pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz",
     ],
     sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",
     strip_prefix = "six-1.10.0",
-    build_file = "third_party/six.BUILD",
+    build_file = "@//third_party:six.BUILD",
 )
 
-new_http_archive(
+http_archive(
     name = "mock_archive",
     urls = [
         "http://mirror.bazel.build/pypi.python.org/packages/a2/52/7edcd94f0afb721a2d559a5b9aae8af4f8f2c79bc63fdbe8a8a6c9b23bbe/mock-1.0.1.tar.gz",
         "https://pypi.python.org/packages/a2/52/7edcd94f0afb721a2d559a5b9aae8af4f8f2c79bc63fdbe8a8a6c9b23bbe/mock-1.0.1.tar.gz",
     ],
     sha256 = "b839dd2d9c117c701430c149956918a423a9863b48b09c90e30a6013e7d2f44f",
     strip_prefix = "mock-1.0.1",
-    build_file = "third_party/mock.BUILD",
+    build_file = "@//third_party:mock.BUILD",
 )
 
-new_http_archive(
+http_archive(
     # NOTE: The name here is used in _enum_module.py to find the sys.path entry.
     name = "enum34_archive",
     urls = [
         "https://mirror.bazel.build/pypi.python.org/packages/bf/3e/31d502c25302814a7c2f1d3959d2a3b3f78e509002ba91aea64993936876/enum34-1.1.6.tar.gz",
         "https://pypi.python.org/packages/bf/3e/31d502c25302814a7c2f1d3959d2a3b3f78e509002ba91aea64993936876/enum34-1.1.6.tar.gz"
         ],
     sha256 = "8ad8c4783bf61ded74527bffb48ed9b54166685e4230386a9ed9b1279e2df5b1",
-    build_file = "third_party/enum34.BUILD"
+    build_file = "@//third_party:enum34.BUILD"
 )

third_party/BUILD

@@ -0,0 +1,2 @@
+# This BUILD file is empty. It is only needed so that this directory is a bazel
+# package and its files can be referenced by `http_archive.build_file`.