PythonPackage doesn't allow version specific setting of EC variables like use_pip

[Flamefire]

The init method of PythonPackage does a lot depending on EC params

For some Python packages with own easyblocks, such as numpy, we may want to use pip starting at a specific version, i.e. set use_pip = True. However the required self.version isn't available until __init__ of EasyBlock finishes, so the derived easyblock must call that first. This however results in PythonPackage setting a lot of member variables already, especially it changes EC variables. Some only work for pip or setup.py:

easybuild-easyblocks/easybuild/easyblocks/generic/pythonpackage.py

Lines 449 to 453 in 9e1cdf2

	self.cfg.update('installopts', '--egg')
	pip_no_index = self.cfg.get('pip_no_index', None)
	if pip_no_index or (pip_no_index is None and self.cfg.get('download_dep_fail')):
	self.cfg.update('installopts', '--no-index')

Hence when the derived EasyBlock wants to/can change use_pip the state has already been irreversibly modified.