__get_norm()__ is called on lines 259 and 265 but it is never imported or defined.
[flake8](http://flake8.pycqa.org) testing of https://github.com/3b1b/manim on Python 3.7.0
$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./old_projects/bell.py:222:38: E999 SyntaxError: invalid syntax
for here, x in (here1, 0), (here2, 4)
^
./old_projects/borsuk.py:217:11: E999 SyntaxError: invalid syntax
y=lambda m: m1.get
^
./old_projects/eoc/chapter6.py:399:35: E999 SyntaxError: invalid syntax
for i, j in (1, 0), (0, 1)
^
./old_projects/nn/network.py:258:16: F821 undefined name 'get_norm'
norm = get_norm(gradient)
^
./old_projects/nn/network.py:264:15: F821 undefined name 'get_norm'
print(get_norm(old_pre_sig_guess - pre_sig_guess))
^
3 E999 SyntaxError: invalid syntax
2 F821 undefined name 'get_norm'
5
```
