Update Python requirements to 3.8+

Python 3.4 is absolutely ancient and 3.8 is also EOL by now, but as long
as it still works we may be interested in continuing to use it(since ST
plugins themselves still runs with it).

Required updating github3.py.

Also, Sublime Text 3 is not the latest anymore, so we just remove the
major version specifier.

README.md

@@ -5,7 +5,7 @@
 [![PyPI](https://img.shields.io/pypi/v/st-package-reviewer.svg)](https://pypi.python.org/pypi/st-package-reviewer)
 [![Python Versions](https://img.shields.io/pypi/pyversions/st-package-reviewer.svg)](https://pypi.python.org/pypi/st-package-reviewer)
 
-A tool to review packages for [Sublime Text 3][]
+A tool to review packages for [Sublime Text][]
 (and its package manager [Package Control][]).
 Supports passing local file paths
 or URLs to GitHub repositories.
@@ -18,7 +18,7 @@ reported by the tool,
 
 ## Installation
 
-Requires **Python 3.4** or higher.
+Requires **Python 3.8** or higher.
 
 ```bash
 $ pip install st-package-reviewer
@@ -62,6 +62,6 @@ Interactive mode:
 ```
 
 
-[Sublime Text 3]: https://sublimetext.com/
+[Sublime Text]: https://sublimetext.com/
 [Package Control]: https://packagecontrol.io/
 [wiki]: https://github.com/packagecontrol/st_package_reviewer/wiki