py3: quote $PYTHON in test-patchbomb.t for Windows

I couldn't get the quoting right in the environment variable, so now it's a
function.

tests/test-patchbomb.t

@@ -22,7 +22,9 b' Mercurial-patchbomb/.* -> Mercurial-patc'
   >     skipblank = False
   >     print(l, end='')
   > EOF
-  $ FILTERBOUNDARY="$PYTHON `pwd`/prune-blank-after-boundary.py"
+  $ filterboundary() {
+  >     "$PYTHON" "$TESTTMP/prune-blank-after-boundary.py"
+  > }
   $ echo "[extensions]" >> $HGRCPATH
   $ echo "patchbomb=" >> $HGRCPATH
 
@@ -357,7 +359,7 b" Test breaking format changes aren't"
 
 test bundle and description:
   $ hg email --date '1970-1-1 0:3' -n -f quux -t foo \
-  >  -c bar -s test -r tip -b --desc description | $FILTERBOUNDARY
+  >  -c bar -s test -r tip -b --desc description | filterboundary
   searching for changes
   1 changesets found
   
@@ -403,7 +405,7 b' with a specific bundle type'
 
   $ hg email --date '1970-1-1 0:3' -n -f quux -t foo \
   >  -c bar -s test -r tip -b --desc description \
-  > --config patchbomb.bundletype=gzip-v1 | $FILTERBOUNDARY
+  > --config patchbomb.bundletype=gzip-v1 | filterboundary
   searching for changes
   1 changesets found
   
@@ -884,7 +886,7 b' test diffstat for multiple patches:'
   
 
 test inline for single patch:
-  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i -r 2 | $FILTERBOUNDARY
+  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i -r 2 | filterboundary
   this patch series consists of 1 patches.
   
   
@@ -927,7 +929,7 b' test inline for single patch:'
 
 
 test inline for single patch (quoted-printable):
-  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i -r 4 | $FILTERBOUNDARY
+  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i -r 4 | filterboundary
   this patch series consists of 1 patches.
   
   
@@ -986,7 +988,7 b' test inline for single patch (quoted-pri'
 
 test inline for multiple patches:
   $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i \
-  >  -r 0:1 -r 4 | $FILTERBOUNDARY
+  >  -r 0:1 -r 4 | filterboundary
   this patch series consists of 3 patches.
   
   
@@ -1138,7 +1140,7 b' test inline for multiple patches:'
   --===*=-- (glob)
 
 test attach for single patch:
-  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a -r 2 | $FILTERBOUNDARY
+  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a -r 2 | filterboundary
   this patch series consists of 1 patches.
   
   
@@ -1189,7 +1191,7 b' test attach for single patch:'
   --===*=-- (glob)
 
 test attach for single patch (quoted-printable):
-  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a -r 4 | $FILTERBOUNDARY
+  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a -r 4 | filterboundary
   this patch series consists of 1 patches.
   
   
@@ -1256,7 +1258,7 b' test attach for single patch (quoted-pri'
   --===*=-- (glob)
 
 test attach and body for single patch:
-  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a --body -r 2 | $FILTERBOUNDARY
+  $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a --body -r 2 | filterboundary
   this patch series consists of 1 patches.
   
   
@@ -1318,7 +1320,7 b' test attach and body for single patch:'
 
 test attach for multiple patches:
   $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -a \
-  >  -r 0:1 -r 4 | $FILTERBOUNDARY
+  >  -r 0:1 -r 4 | filterboundary
   this patch series consists of 3 patches.
   
   
@@ -1775,7 +1777,7 b' tagging csets:'
 
 test inline for single named patch:
   $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i \
-  >   -r 2 | $FILTERBOUNDARY
+  >   -r 2 | filterboundary
   this patch series consists of 1 patches.
   
   
@@ -1818,7 +1820,7 b' test inline for single named patch:'
 
 test inline for multiple named/unnamed patches:
   $ hg email --date '1970-1-1 0:1' -n -f quux -t foo -c bar -s test -i \
-  >    -r 0:1 | $FILTERBOUNDARY
+  >    -r 0:1 | filterboundary
   this patch series consists of 2 patches.
   
   
@@ -2124,7 +2126,7 b' test single flag for single patch (and n'
   $ hg up -qr1
   $ echo dirt > a
   $ hg email --date '1970-1-1 0:1' -n --flag fooFlag -f quux -t foo -c bar -s test \
-  >  -r 2 | $FILTERBOUNDARY
+  >  -r 2 | filterboundary
   this patch series consists of 1 patches.