bump version to v0.18.1b0
# changes with bug fixes, maybe also minor API changes
#
# release_state:
-# a .. alpha: adding new features - non public development state
+# a .. alpha: adding new features - non-public development state
# b .. beta: testing new features - public development state
# rc .. release candidate: testing release - public testing
# release: public release
# 1. bug fix release beta0: VERSION = "0.9.1b0"; version = (0, 9, 1, 'b0')
# 2. bug fix release: VERSION = "0.9.2"; version = (0, 9, 2, 'release')
-version = (0, 18, 0, "release")
-__version__ = "0.18"
+version = (0, 18, 1, "b0")
+__version__ = "0.18.1b0"
