Merge pull request matplotlib#87 from QuLogic/prep39

MNT: Update version to 3.9.0rc1

Author: ksunden

mpl_sphinx_theme/_version.py

@@ -5,4 +5,4 @@
 # Distributed under the terms of the Modified BSD License.
 
 version_info = (3, 9, 0)
-__version__ = ".".join(map(str, version_info)) + "dev0"
+__version__ = ".".join(map(str, version_info)) + "rc1"