README.rst: Update pypi links

Author: Matthias Koeppe

README.rst

@@ -44,7 +44,7 @@ The available objects and functions from the `ppl` Python module are:
 Installation
 ------------
 
-The project is available at `Python Package Index <https://pypi.python.org/pypi/pplpy/>`_ and
+The project is available at `Python Package Index <https://pypi.org/project/pplpy/>`_ and
 can be installed with pip::
 
     $ pip install pplpy
@@ -96,9 +96,9 @@ Requirements
 
 - `Cython <http://cython.org>`_ (tested with both 0.29 and 3.0)
 
-- `cysignals <https://pypi.python.org/pypi/cysignals>`_
+- `cysignals <https://pypi.org/project/cysignals>`_
 
-- `gmpy2 <https://pypi.python.org/pypi/gmpy2>`_
+- `gmpy2 <https://pypi.org/project/gmpy2>`_
 
 On Debian/Ubuntu systems the dependencies can be installed with::