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 in module Main must be qualified
ERROR: UndefVarError: `step` not defined
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@d17c1d5). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #20   +/-   ##
=======================================
  Coverage        ?   16.03%           
=======================================
  Files           ?        5           
  Lines           ?      449           
  Branches        ?        0           
=======================================
  Hits            ?       72           
  Misses          ?      377           
  Partials        ?        0           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[stevengj]

Could you make a corresponding PR for PyPlot.jl?