Encode hard coded version in one location instead of two

This could be improved to go full fledged versioneer like BuildStream, but
for now instead of hard coding the version in setup.py and doc/source/conf.py,
just encode it in the toplevel __init__.py and derive it from both locations.

 * src/buildstream_plugins/__init__.py: Set __version__ here

 * setup.py: Import __version__ and use it for invoking setuptools

 * doc/source/conf.py: Import __version__ and use it to set the docs version
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 1c0a614..563fed7 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -19,6 +19,8 @@
 #
 import os
 import sys
+from buildstream_plugins import __version__
+
 sys.path.insert(0, os.path.abspath('..'))
 
 # -- General configuration ------------------------------------------------
@@ -62,9 +64,9 @@
 # built documents.
 #
 # The short X.Y version.
-version = '1.91'
+version = __version__
 # The full version, including alpha/beta/rc tags.
-release = '1.91.0'
+release = __version__
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
diff --git a/setup.py b/setup.py
index c1fcb66..bf28027 100755
--- a/setup.py
+++ b/setup.py
@@ -37,10 +37,16 @@
 ) as readme:
     long_description = readme.read()
 
+###############################################################################
+#                             Load the version                                #
+###############################################################################
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "src"))
+from buildstream_plugins import __version__  # pylint: disable=wrong-import-position
+
 
 setup(
     name="buildstream-plugins",
-    version="1.95.1",
+    version=__version__,
     author="BuildStream Developers",
     author_email="dev@buildstream.apache.org",
     classifiers=[
diff --git a/src/buildstream_plugins/__init__.py b/src/buildstream_plugins/__init__.py
index e69de29..e0d37ee 100644
--- a/src/buildstream_plugins/__init__.py
+++ b/src/buildstream_plugins/__init__.py
@@ -0,0 +1,4 @@
+#
+# Remember to adjust this version number before tagging releases
+#
+__version__ = "1.95.1"