Closing figures is done by number

Currently, closing a figure is done using the num attribute in the manager. This can cause problems if there are figures not managed by pyplot, as the number could be the same as a
pyplot-managed figure.

As there are efforts to reduce the dependency on pyplot, such as #14024, this should be changed for those to work well. Alternatively, a way to create a manager which selects an appropiate number could work.

Code for reproduction

import matplotlib.pyplot as plt 

# Figure managed by pyplot 
fig1 = plt.figure() 
 
# Figure not managed by pyplot, but using the same backend 
fig2 = fig = plt.new_figure_manager(1).canvas.figure 

fig1.show() 
fig2.show()

Actual outcome

Closing one figure closes both.

Expected outcome

Closing one figure leaves the other open.

Matplotlib version

• Operating system: All
• Matplotlib version: 3.0.3
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: All