Update Python version in sample conda environment build to Python 3.9 (#740)

Author: willschlitzer

doc/install.rst

@@ -77,7 +77,7 @@ First, we must configure conda to get packages from the
 Now we can create a new conda environment with Python and all our dependencies installed
 (we'll call it ``pygmt`` but you can change it to whatever you want)::
 
-     conda create --name pygmt python=3.8 pip numpy pandas xarray netcdf4 packaging gmt
+     conda create --name pygmt python=3.9 pip numpy pandas xarray netcdf4 packaging gmt
 
 Activate the environment by running::