Support calling PyCall.npyinitialize() during sysimage compilation

In PR JuliaPy#651, I introduced code to empty `npy_api` in `__init__` but
forgot to reset `npy_initialized`.  This introduces a bug if
`npyinitialize` is called during sysimage compilation (`--output-o`
mode) because the main body of `npyinitialize` is not run at run-time.

Author: tkf

aot/precompile.jl

@@ -14,3 +14,15 @@ isdefined(MacroTools, :__init__) && MacroTools.__init__()
 using PyCall
 PyCall.__init__()
 PyCall.pyimport("sys")[:executable]
+
+has_numpy = try
+    PyCall.pyimport("numpy")
+    true
+catch
+    false
+end
+if has_numpy
+    # Invoke numpy support (which mutates various states in PyCall) to
+    # test that it does not introduce any run-time bugs:
+    PyCall.npyinitialize()
+end

src/pyinit.jl

@@ -131,6 +131,7 @@ function __init__()
     _finalized[] = false
     empty!(_namespaces)
     empty!(eventloops)
+    global npy_initialized = false
     empty!(npy_api)
     empty!(pycall_gc)
     empty!(pyexc)