Update signature of bokeh.io.export.export_png

[Guillemdb]

The bokeh.io.export.export.export_png function has been updated, and now filename needs to be passes as a keyword argument.

• This PR updates the filename parameter passed to the function to reflect the update of the export_png signature.

[codecov]

Codecov Report

Merging #1165 into master will not change coverage by %.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1165   +/-   ##
=======================================
  Coverage   87.81%   87.81%           
=======================================
  Files         105      105           
  Lines       12591    12591           
=======================================
  Hits        11057    11057           
  Misses       1534     1534           

Impacted Files	Coverage Δ
panel/io/save.py	82.43% <0.00%> (ø)

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[philippjfr]

Thanks @Guillemdb!

[mattpap]

As an FYI, all APIs in bokeh will get keyword only args (if needed) in 3.0. I think it's a good idea to start writing code now that uses the APIs as if this was done already. This will simplify transition to the next major version.

[philippjfr]

Thanks for the heads up.