GCPy 1.3.0 pre-release items

CHANGELOG.md
- Add version number and release data

setup.py
- Change version number to 1.3.0 (MAJOR=1, MINOR=3, MICRO=0)
- Also remove Python 3.6, 3.7, 3.8 text; GCPy works best with Python 3.9

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

CHANGELOG.md

@@ -5,6 +5,9 @@ All notable changes to GCPy will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased
+
+## [1.3.0] -- 2022-10-25
+
 ### Added
 - New features in benchmarking scripts (@lizziel, @yantosca)
   - Force garbage collection at end benchmarking functions (@yantosca)

setup.py

@@ -24,14 +24,12 @@
     'Operating System :: OS Independent',
     'Intended Audience :: Science/Research',
     'Programming Language :: Python',
-    'Programming Language :: Python :: 3.6',
-    'Programming Language :: Python :: 3.7',
-    'Programming Language :: Python :: 3.8',
+    'Programming Language :: Python :: 3.9',
     'Topic :: Scientific/Engineering',
 ]
 
 MAJOR = 1
-MINOR = 2
+MINOR = 3
 MICRO = 0
 EXTRA = '' # for alpha (aN), beta (bN), rc (rcN) versions