Bump notebook from 5.7.8 to 6.4.12 in /assignment1

Bumps [notebook](http://jupyter.org) from 5.7.8 to 6.4.12.

---
updated-dependencies:
- dependency-name: notebook
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

assignment1/requirements.txt

@@ -32,7 +32,7 @@ mistune==0.8.4
 nbconvert==5.4.1
 nbdime==1.0.5
 nbformat==4.4.0
-notebook==5.7.8
+notebook==6.4.12
 numpy==1.16.2
 pandocfilters==1.4.2
 parso==0.3.4