* Support a "is_global" and "is_private" member on Rules.

When writing linters it is currently impossible to know (via rule introspection)
if a given rule is private or global. We have banned global rules for our use
case and we have to resort to a janky regex against our rules files to know if
anyone is about to commit a global rule. I figure exposing these two flags via
python will be useful for programatically checking those bits.

I'm not very pleased with the name "is_global" - I wanted to go with just
"global" and "private" but "global" is a reserved keyword and rule.global breaks
the python interpreter. I'm open to changing the member names if you have any
suggestions.

* Decrement reference counts on global and private.

* Update global and private checks after API changes.

* Upgrade YARA submodule and adapt to new API.

* Upgrade YARA and adapt to API changes.

* Fix build in Appveyor (#131)

* The precompiled OpenSSL is now extracted from the NuGet package generated by https://ci.appveyor.com/project/plusvic/openssl.

* OpenSSL was upgraded to version 1.1.1.

* Handle invalid unicode in metadata values.

In #135 it was brought up that you can crash the python interpreter if you have
invalid unicode in a metadata value. This is my attempt to fix that by
attempting to create a string, and if that fails falling back to a bytes object.
On the weird chance that the bytes object fails to create I added a safety check
so that we don't add a NULL ptr to the dictionary (this is how the crash was
manifesting).

It's debatable if we want to ONLY add strings as metadata, and NOT fallback to
bytes. If we don't fall back to bytes the only other option I see is to silently
drop that metadata on the floor. The tradeoff here is that now you may end up
with a string or a bytes object in your metadata dictionary, which is less than
ideal IMO.

I'm open to suggestions on this one.

Fixes #135

* Add error handling to conversion to Unicode
Metadata test accepts stripped or original characters

* Remove 'or' clause from tests and add another NULL test check.

Co-authored-by: malvidin <malvidin@gmail.com>

fix #123

Remove Python versions that are not supported anymore and add new ones.