Add missing `Pkg` import before `PackageSpec`

pysr/julia_helpers.py

@@ -200,6 +200,7 @@ def init_julia(julia_project=None, quiet=False):
 
 
 def _add_sr_to_julia_project(Main, io_arg):
+    Main.eval("using Pkg")
     Main.sr_spec = Main.PackageSpec(
         name="SymbolicRegression",
         url="https://github.com/MilesCranmer/SymbolicRegression.jl",