allow building sysimage with custom PyCall

[marius311]

The current sysimage just installs the latest PyCall from the registry. This adds a --pycall-env option which lets you bake in an existing one from any project you want,

$ python -m julia.sysimage --pycall-env . sys.so # take PyCall from the project in the current folder

I found this useful in my debugging / developing, and I think it'd be nice to have in the official repo.

[codecov]

Codecov Report

Merging #422 into master will decrease coverage by 0.10%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
- Coverage   87.48%   87.38%   -0.11%     
==========================================
  Files          39       39              
  Lines        2246     2251       +5     
==========================================
+ Hits         1965     1967       +2     
- Misses        281      284       +3     

Impacted Files	Coverage Δ
src/julia/sysimage.py	72.36% <50.00%> (-2.28%)	⬇️

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[tkf]

Thanks! Yes, it'd be nice to have.

Can you add a test for this? You can put it in https://github.com/tkf/pyjulia/blob/pyjuliatester/src/julia/tests/test_sysimage.py

Also, I wonder if it makes sense to call --pycall-env something else. For example, it might be useful to list, e.g., PyPlot and Plots in such an environment (even though currently they are not get loaded). Maybe --sysimage-env? Though --pycall-env kinda still makes sense since we always have PyCall in it.

Other than that, it LTGM!

[mkitti]

Did we ever add a test?