Fix #927, add encoding=utf-8 on file write

[davidlatwe]

This PR will allow installing/releasing packages which has Unicode character in package.py.

For example, this relaxing package "☺️" would failed before this PR

name = "_releaxed"

version = "0.1.0"

authors = ["☺️"]

build_command = False


def commands():
    print("Releax ! ☺️")

But after this PR, you get

C:\Users\david\dev\test\unicode-content>rez-build --install

================================================================================
Building _releaxed-a.b.c...
================================================================================
Resolving build environment:
resolved by david@DESKTOP-EABK8SP, on Mon Aug 10 02:34:17 2020, using Rez v2.63.0

requested packages:
~platform==windows           (implicit)
~arch==AMD64                 (implicit)
~os==windows-10.0.19041.SP0  (implicit)

resolved packages:

Invoking custom build system...

================================================================================
Build Summary
================================================================================

All 1 build(s) were successful.


C:\Users\david\dev\test\unicode-content>rez-env _releaxed
Releax ! ☺️

You are now in a rez-configured environment.