Remove PySCF<2.10 constraint

[awvwgk]

Before PySCF 2.10

• density fitting would always use jkfit auxbasis independent of whether xc functional is hybrid or not

PySCF 2.10 or later

• auxbasis now detected based on whether xc functional is hybrid or not, jfit used for non-hybrids and jkfit used for hybrids

Updates:

• specify auxbasis explicitly in tests where the auxbasis change will give a significant change in result
• remove constraint on PySCF version (maybe add PySCF<2.12 just to be safe?)
• test PySCF<2.10 and PySCF>=2.10 in CI
• add a warning if skala is used with auxbasis=None?

Closes #16