Address Conda-build issue

This release exists solely to resolve a Conda-build issue as discussed on #740. If you're not actively working on Conda packaging for Miller, this release has no added value for you above 5.10.2.

Likewise, there's no Windows mlr.exe for this final (technical & specific) Miller 5.x release -- for Miller 6.0.0 (coming soon!) and above there will be mlr.exe as a reliably standard part of each release.

Also note that the tarball is named miller-5.10.3.tar.gz, in contrast to mlr-5.10.2.tar.gz and likewise for all earlier releases. This is being done for forward compatibility with Miller 6.0.0 and beyond which will use names of the form miller-6.0.0.tar.gz, as proposed in #360.