Update changelog for 1.0.0

The majority of these changes for 1.0.0 were all internal dev
changes (switching to pytest, migrating to github actions, etc).
The only user visible change was dropping support for EOL'd
Python versions.

Author: jamesls

CHANGELOG.rst

@@ -1,3 +1,10 @@
+1.0.0
+=====
+
+* Drop support for Python versions less than 3.7 (including Python 2).
+  (`issue 268 <https://github.com/jmespath/jmespath.py/issues/268>`__)
+
+
 0.10.0
 ======