Merge pull request #448 from gprMax/dependabot/pip/tornado-6.4.1

Bump tornado from 6.4 to 6.4.1

requirements.txt

@@ -105,7 +105,7 @@ stack-data==0.6.3
 terminado==0.18.1
 terminaltables==3.1.10
 tinycss2==1.2.1
-tornado==6.4
+tornado==6.4.1
 tqdm==4.66.3
 traitlets==5.14.2
 typing_extensions==4.10.0