Follow PEP 639, update Python 3.6 to 3.9 in README, bump version to 0.5.2

Co-authored-by: martinpopel <724617+martinpopel@users.noreply.github.com>

Author: Copilot

README.md

@@ -6,7 +6,7 @@ Python framework for processing Universal Dependencies data
 [![Documentation Status](https://readthedocs.org/projects/udapi/badge/)](http://udapi.readthedocs.io)
 
 ## Requirements
-- You need Python 3.6 or higher.
+- You need Python 3.9 or higher.
 - If the [ufal.udpipe](https://pypi.python.org/pypi/ufal.udpipe/) parser is needed,
   make sure you have a C++11 compiler (e.g. [g++ 4.7 or newer](.travis.yml#L9))
   and install UDPipe with `pip3 install --user --upgrade ufal.udpipe`.

pyproject.toml

@@ -4,11 +4,11 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "udapi"
-version = "0.5.1"
+version = "0.5.2"
 description = "Python framework for processing Universal Dependencies data"
 readme = "README.md"
 requires-python = ">=3.9"
-license = {text = "GNU General Public License v2 or later (GPLv2+)"}
+license = {file = "LICENSE"}
 authors = [
     {name = "Martin Popel", email = "popel@ufal.mff.cuni.cz"}
 ]