Merge branch 'increase_python_version' into 'NIFTy_6'

Increase python version

See merge request !466

ChangeLog

 Changes since NIFTy 5:
 
+Minimum Python version increased to 3.6
+=======================================
+
+
 FFT convention adjusted
 =======================
 

README.md

@@ -45,7 +45,7 @@ Installation
 
 ### Requirements
 
-- [Python 3](https://www.python.org/) (3.5.x or later)
+- [Python 3](https://www.python.org/) (3.6.x or later)
 - [SciPy](https://www.scipy.org/)
 
 Optional dependencies:

setup.py

@@ -41,7 +41,7 @@ setup(name="nifty6",
       license="GPLv3",
       setup_requires=['scipy>=1.4.1', 'numpy>=1.17'],
       install_requires=['scipy>=1.4.1', 'numpy>=1.17'],
-      python_requires='>=3.5',
+      python_requires='>=3.6',
       classifiers=[
         "Development Status :: 4 - Beta",
         "Topic :: Utilities",