Set minimum Python version to 3.8 in README.rst

Patches #1676

Author: weiji14

README.rst

@@ -218,7 +218,7 @@ Compatibility with GMT/Python/NumPy versions
     * - `Dev <https://github.com/GenericMappingTools/pygmt/milestone/9>`_ (upcoming release)
       - `Dev Documentation <https://www.pygmt.org/dev>`_ (reflects `main branch <https://github.com/GenericMappingTools/pygmt>`_)
       - >=6.3.0
-      - >=3.7
+      - >=3.8
       - >=1.19
     * - `v0.5.0 <https://github.com/GenericMappingTools/pygmt/releases/tag/v0.5.0>`_ (latest release)
       - `v0.5.0 Documentation <https://www.pygmt.org/v0.5.0>`_