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

Compilation of `plyvel` 1.0.5 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 is available in Ubuntu 20.04, and 1.3.0 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__,