Now that Mercurial supports Python 3, we no longer have to use a

dependency specifier to limit installs to Python 2.7. We can instead

just simply install. We're keeping the original minimum version for now

(making it easier to test with Mercurial 4.x), but this will always get

the 5.x release on Python 3.x since there won't be an existing 4.x

release installed.
This change also works around the lack of pre-compiled Python 3.8

p4python builds on non-Linux platforms. If a pre-compiled build is not

available, then pip will try to install the p4python source tarball,

which cannot work by default on non-Linux platforms due to a reliance on

manual configuration of the OpenSSL location. Until they either improve

upon this or provide 3.8 builds, we'll need to skip installation on this

version of Python.
The Perforce unit tests received updates to run without having P4

installed. We had some old code that attempted to do this, but it was

archaic and no longer worked.
And finally, subvertpy 0.10.1 (the latest release, which is now 2.5

years old) fails when imported on Python 3.8. There's no indicator as to

when a new release may be, and it's pretty broken on 3.8, so we no

longer attempt to install it there.

Tested installing on Python 2.7 and 3.5-3.8.
Verified that Mercurial 5.3 (latest series) was installed on all

Python 3.x environments.
Verified that p4python was installed on all Python environments aside

from 3.8.
Verified that subvertpy was not installed on Python 3.8.
All unit tests pass on Python 3.8.