Apply suggestions from code review

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

Author: willschlitzer

README.rst

@@ -229,8 +229,8 @@ Compatibility with Python and GMT versions
 ======= ========== =========
 PyGMT   GMT        Python
 ======= ========== =========
-0.3.0   >=6.1.1    >=3.6
-0.2.1   >=6.1.1    3.6 - 3.9
+0.3.0   >=6.1.1    >=3.7
+0.2.1   >=6.1.1    >=3.6
 0.2.0   >=6.1.1    3.6 - 3.8
 0.1.x   >=6.0.0    3.6 - 3.8
 ======= ========== =========