Add support for upgrading to 2.6

[chriskuehl]

This allows the upgrader to upgrade Baseplate repos to the newly-released 2.6.0 version.

I also updated the README to recommend using pipx for installing and running this tool inside an isolated virtualenv, or alternatively to install it inside a virtualenv manually. I don't think we should recommend doing pip install X with a global/user Python installation since this almost always causes problems later on.

I tested this on a service and can see it updates the baseplate version:

$ baseplate.py-upgrader .
Baseplate.py Upgrader
Upgrading .
Python version: 3.8
Current version: v2.3.3
Target version: v2.6.0 (2.6 series)

 ✓ Updated baseplate to 2.6.0 in requirements.txt

Automatic upgrade successful!
There's more to do:
 • Review the logs above.
 • Review the diff in your application.
 • Apply code formatters to clean up refactored code.
 • Thoroughly test your application.
 • Commit the changes.

$ git diff HEAD
diff --git a/requirements.txt b/requirements.txt
index ea2f28c..52cfa0e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 alembic==1.7.6
 PasteDeploy==2.0.1
 PyJWT==2.4.0
-baseplate==2.3.3
+baseplate==2.6.0
 certifi==2023.7.22
 chardet==3.0.4
 gevent==23.9.0