Make version number pep 440 compliant

Author: Mariko Wakabayashi

jupyter_server/_version.py

@@ -7,7 +7,7 @@
 # the version string is propagated to the JavaScript files,  do not forget to
 # patch the JavaScript files in `.postN` release done by distributions.
 
-# Next beta/alpha/rc release: The version number for beta is X.Y.ZbN **without dots**.
+# Next beta/alpha/rc release: The version number for beta is X.Y.Z.bN
 
-version_info = (1, 1, 0, '')
-__version__ = '.'.join(map(str, version_info[:3])) + ''.join(version_info[3:])
+version_info = (1, 1, 0, 'dev0')
+__version__ = '.'.join(map(str, version_info))