Defer import of matplotlib until needed

[bdraco]

Similar to how cartopy and geoip2 are handled, defer import of matplotlib until needed.

Testing with import scapy.packet

Before:
bdraco 52845 0.0 0.2 409457040 61264 s004 T 10:00AM 0:00.22 /opt/homebrew/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/Resources/Python.app/Contents/MacOS/Python

After:
bdraco 52900 0.0 0.1 409195664 46368 s004 T 10:01AM 0:00.17 /opt/homebrew/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/Resources/Python.app/Contents/MacOS/Python

fixes #3578

matplotlib has a heavy dep chain

I did manual testing to make sure I could still generate images when matplotlib installed, and verified the warning at -H startup still works when its not

[codecov]

Codecov Report

Merging #3579 (8e753ef) into master (285824f) will increase coverage by 35.38%.
The diff coverage is 92.85%.

@@             Coverage Diff             @@
##           master    #3579       +/-   ##
===========================================
+ Coverage   50.80%   86.18%   +35.38%     
===========================================
  Files         171      285      +114     
  Lines       40155    64883    +24728     
===========================================
+ Hits        20399    55922    +35523     
+ Misses      19756     8961    -10795     

Impacted Files	Coverage Δ
scapy/libs/test_pyx.py	76.19% <ø> (ø)
scapy/layers/inet.py	67.05% <50.00%> (+44.81%)	⬆️
scapy/plist.py	83.50% <83.33%> (+65.77%)	⬆️
scapy/libs/matplot.py	100.00% <100.00%> (ø)
scapy/packet.py	82.13% <100.00%> (+50.77%)	⬆️
scapy/contrib/automotive/gm/gmlan_scanner.py	84.51% <0.00%> (ø)
scapy/contrib/automotive/obd/pid/pids_60_7F.py	100.00% <0.00%> (ø)
scapy/contrib/automotive/obd/mid/__init__.py	100.00% <0.00%> (ø)
scapy/contrib/scada/__init__.py	100.00% <0.00%> (ø)
scapy/layers/usb.py	71.54% <0.00%> (ø)
... and 245 more

[bdraco]

Thanks !

[gpotter2]

Sorry for the delay.

• Could you move the new file to scapy/libs/ ?
• Would you mind doing the same for PYX to remove extlib the remaining entirely?

Would be awesome, thanks a lot

[bdraco]

Retested to make sure I could still generate a plot with a.plot(lambda x:x[1].id)

[bdraco]

I've tried so many incantations with py27 but haven't been able to get it to patch correctly

[bdraco]

Should be good now. There was a name collision in the first refactor that py27 couldn't handle since it like has . in its load path.

[gpotter2]

Thanks for the PR !