setup.py: use plyvel==1.2.0 instead of 1.0.5

Compilation of `plyvel` 1.0.5 (July 17, 2018)
proceeds when using Python 3.7 and 3.8, but fails with 3.9.

If using `plyvel` 1.2 or 1.3, no compilation is necessary,
as the packages are downloaded from `pip` without issue.

Version 1.2.0 (January 22, 2020) is available in Ubuntu 20.04,
and 1.3.0 (October 10, 2020) is available in Arch,
so these are good versions to use as reference.

setup.py

@@ -9,7 +9,7 @@
 
 PLYVEL = []
 if sys.platform.startswith('linux'):
-    PLYVEL.append('plyvel==1.0.5')
+    PLYVEL.append('plyvel==1.2.0')
 
 setup(
     name=__name__,