Increase python requirement to >= 3.8 (pvlib#2029)

* require python >=3.8

* update CI configurations

* whatsnew

* remove pre-3.8 cruft

* fix h5py/numpy versioning issue for py3.8-min

* update gallery examples for newer pandas

* update asv config for python 3.8

* fix asv config syntax error

* one more asv config edit

Author: kandersolar

docs/sphinx/source/whatsnew/v0.10.5.rst

@@ -51,6 +51,7 @@ Documentation
 Requirements
 ~~~~~~~~~~~~
 * Minimum version of Python increased from 3.7 to 3.8. (:issue:`1975`, :pull:`2029`)
+* Python 3.8 or higher. (:issue:`1975`, :pull:`2029`)
 * Minimum version of scipy advanced from 1.5.0 to 1.6.0. (:pull:`2027`)