don't export step

[t-bltg]

According to https://github.com/stevengj/PythonPlot.jl#differences-from-pyplotjl, step is renamed to plotstep to avoid piracy, but it is still exported, causing the following:

julia>using PythonPlot julia> r = range(0, 1; length=10) julia> step(r) WARNING: both PythonPlot and Base export"step"; uses of it inmodule Main must be qualified ERROR: UndefVarError:`step` not defined Stacktrace: [1] top-level scope @ REPL[4]:1

[codecov-commenter]

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[stevengj]

Could you make a corresponding PR for PyPlot.jl?