Open figures using the associated application on Windows (#952)

This PR improves the `launch_external_viewer` function to open images
using the associated application (just like double-click the images) on
Windows.

The implemention is done by calling the
[`os.startfile`](https://docs.python.org/3/library/os.html#os.startfile)
function.

`os.startfile` is only available on Windows, so need to disable the pylint
error `no-member`.

The code was originall written in #269 by @leouieda, re-used in #529.

pygmt/helpers/utils.py

@@ -1,6 +1,7 @@
 """
 Utilities and common tasks for wrapping the GMT modules.
 """
+import os
 import shutil
 import subprocess
 import sys
@@ -195,8 +196,9 @@ def launch_external_viewer(fname):
     """
     Open a file in an external viewer program.
 
-    Uses the ``xdg-open`` command on Linux, the ``open`` command on macOS, and
-    the default web browser on other systems.
+    Uses the ``xdg-open`` command on Linux, the ``open`` command on macOS, the
+    associated application on Windows, and the default web browser on other
+    systems.
 
     Parameters
     ----------
@@ -214,8 +216,10 @@ def launch_external_viewer(fname):
         subprocess.run(["xdg-open", fname], check=False, **run_args)
     elif os_name == "darwin":  # Darwin is macOS
         subprocess.run(["open", fname], check=False, **run_args)
+    elif os_name == "win32":
+        os.startfile(fname)  # pylint: disable=no-member
     else:
-        webbrowser.open_new_tab("file://{}".format(fname))
+        webbrowser.open_new_tab(f"file://{fname}")
 
 
 def args_in_kwargs(args, kwargs):