Port regress-python3-mangle.mk to Solaris "sed", redux.

Per experimentation and buildfarm failures, Solaris' "sed"
has got some kind of problem with regexes that use both '*'
and '[[:alpha:]]'.  We can work around that by replacing
'[[:alpha:]]' with '[a-zA-Z]', which is plenty good enough
for our purposes, especially since this is only needed in
long-stable branches.

I chose to flat-out remove the second pattern of this sort,
's/except \([a-zA-Z][a-zA-Z.]*\), *\([a-zA-Z][a-zA-Z]*\):/except \1 as \2:/g'
because we haven't needed it since 8.4.

Follow-on to c3556f6fa, which probably missed catching this
because the problematic pattern was already gone when that
patch was written.

Patch v10-v12 only, as the problem manifests only there.
We have a line of dead code in v13-v14, which isn't worth
changing, and the whole mess is gone as of v15.

Discussion: https://postgr.es/m/165561.1661984701@sss.pgh.pa.us

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