feat: expose library version at google.auth.__version (#683)

Move the version from `setup.py` to `google/auth/version.py`.  Same as googleapis/python-api-core#80.  (see googleapis/python-api-core#27 for motivation). This is option 3 in https://packaging.python.org/guides/single-sourcing-package-version/.

This unblocks a version check I'd like to add in googleapis/python-api-core#134.

Usage:

```py
>>> import google.auth
>>> google.auth.__version__
'1.25.0'
```

google/auth/__init__.py

@@ -16,8 +16,13 @@
 
 import logging
 
+from google.auth import version as google_auth_version
 from google.auth._default import default, load_credentials_from_file
 
+
+__version__ = google_auth_version.__version__
+
+
 __all__ = ["default", "load_credentials_from_file"]
 
 # Set default logging handler to avoid "No handler found" warnings.

google/auth/version.py

@@ -0,0 +1,15 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+__version__ = "1.27.1"

setup.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import io
+import os
 
 from setuptools import find_packages
 from setuptools import setup
@@ -37,7 +38,12 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.27.1"
+package_root = os.path.abspath(os.path.dirname(__file__))
+
+version = {}
+with open(os.path.join(package_root, "google/auth/version.py")) as fp:
+    exec(fp.read(), version)
+version = version["__version__"]
 
 setup(
     name="google-auth",