clean up package build process and move from pdm back to setuptools

require keras~=2.0

Bugfix: add back __version__ property

##Changelog

• use pdm for build
• add minimal tests

Changelog

• update dependencies
• clean up
• switch to micromamba tests + conda-forge build

pyapetnet v1.5

changelog vs v1.4.1

• add command line options to specify series description (suffix and prefix) when predicting from dicom
• use tensorflow instead of tensorflow-cpu as dependency

pyapetnet v1.4.1

changelog vs v1.3.2

• use SimpleITK to image registration

##change log vs v1.3.1

• require only tensorflow-cpu by default

Changelog vs v1.3

• update of installation instructions

pyapetnet v1.3

changes vs v1.2

• only minor code reformatting (yapf) and restructuring changes), no changes for the user interface
• removal of obsolete TF v1.x code