Can not install PyPlot.jl

```
julia> import PyPlot
ERROR: InitError: PyError (PyImport_ImportModule

The Python package matplotlib could not be found by pyimport. Usually this means
that you did not install matplotlib in the Python version being used by PyCall.

PyCall is currently configured to use the Python version at:

C:/Users/chang/.julia/conda/3/python.exe

and you should use whatever mechanism you usually use (apt-get, pip, conda,
etcetera) to install the Python package containing the matplotlib module.

One alternative is to re-configure PyCall to use a different Python
version on your system: set ENV["PYTHON"] to the path/name of the python
executable you want to use, run Pkg.build("PyCall"), and re-launch Julia.

Another alternative is to configure PyCall to use a Julia-specific Python
distribution via the Conda.jl package (which installs a private Anaconda
Python distribution), which has the advantage that packages can be installed
and kept up-to-date via Julia. As explained in the PyCall documentation,
set ENV["PYTHON"]="", run Pkg.build("PyCall"), and re-launch Julia. Then,
To install the matplotlib module, you can use `pyimport_conda("matplotlib", PKG)`,
where PKG is the Anaconda package the contains the module matplotlib,
or alternatively you can use the Conda package directly (via
`using Conda` followed by `Conda.add` etcetera).

)
ModuleNotFoundError("No module named 'matplotlib'",)

Stacktrace:
[1] pyimport(::String) at C:\Users\chang\.julia\packages\PyCall\0jMpb\src\PyCall.jl:486
[2] pyimport_conda(::String, ::String, ::String) at C:\Users\chang\.julia\packages\PyCall\0jMpb\src\PyCall.jl:650
[3] pyimport_conda at C:\Users\chang\.julia\packages\PyCall\0jMpb\src\PyCall.jl:649 [inlined]
[4] __init__() at C:\Users\chang\.julia\packages\PyPlot\mQXSC\src\init.jl:174
[5] _include_from_serialized(::String, ::Array{Any,1}) at .\loading.jl:630
[6] macro expansion at .\logging.jl:312 [inlined]
[7] _require_search_from_serialized(::Base.PkgId, ::String) at .\loading.jl:701
[8] _require(::Base.PkgId) at .\loading.jl:934
[9] require(::Base.PkgId) at .\loading.jl:855
[10] macro expansion at .\logging.jl:311 [inlined]
[11] require(::Module, ::Symbol) at .\loading.jl:837
during initialization of module PyPlot
```