fix: use version.py instead of pkg_resources.get_distribution (#80)

* fix: use version.py instead of pkg_resources.get_distribution
diff --git a/google/api_core/__init__.py b/google/api_core/__init__.py
index c762e18..605dd8b 100644
--- a/google/api_core/__init__.py
+++ b/google/api_core/__init__.py
@@ -17,7 +17,6 @@
 This package contains common code and utilties used by Google client libraries.
 """
 
-from pkg_resources import get_distribution
+from google.api_core import version as api_core_version
 
-
-__version__ = get_distribution("google-api-core").version
+__version__ = api_core_version.__version__
diff --git a/google/api_core/client_info.py b/google/api_core/client_info.py
index b196b7a..6c04d5d 100644
--- a/google/api_core/client_info.py
+++ b/google/api_core/client_info.py
@@ -22,8 +22,10 @@
 
 import pkg_resources
 
+from google.api_core import version as api_core_version
+
 _PY_VERSION = platform.python_version()
-_API_CORE_VERSION = pkg_resources.get_distribution("google-api-core").version
+_API_CORE_VERSION = api_core_version.__version__
 
 try:
     _GRPC_VERSION = pkg_resources.get_distribution("grpcio").version
diff --git a/google/api_core/version.py b/google/api_core/version.py
new file mode 100644
index 0000000..f49e533
--- /dev/null
+++ b/google/api_core/version.py
@@ -0,0 +1,15 @@
+# Copyright 2020 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__ = "1.22.3"
diff --git a/setup.py b/setup.py
index e118966..a35121e 100644
--- a/setup.py
+++ b/setup.py
@@ -22,7 +22,12 @@
 
 name = "google-api-core"
 description = "Google API client core library"
-version = "1.22.3"
+
+version = {}
+with open("google/api_core/version.py") as fp:
+    exec(fp.read(), version)
+version = version['__version__']
+
 # Should be one of:
 # 'Development Status :: 3 - Alpha'
 # 'Development Status :: 4 - Beta'