Fix stackless-3.7.5 python-build script (#1616)

Remove extra whitespace in quoted package_url#checksum string.

Prevent the following error:

  unexpected checksum length: 65 (e2562a8d235adc19be5451c170837f53ef916aec4cd5cd17d9e0ab1f1b875d3f )
  expected 0 (no checksum), 32 (MD5), or 64 (SHA2-256)

plugins/python-build/share/python-build/stackless-3.7.5

@@ -1,4 +1,4 @@
 #require_gcc
 install_package "openssl-1.0.2k" "https://www.openssl.org/source/old/1.0.2/openssl-1.0.2k.tar.gz#6b3977c61f2aedf0f96367dcfb5c6e578cf37e7b8d913b4ecb6643c3cb88d8c0" mac_openssl --if has_broken_mac_openssl
 install_package "readline-8.0" "https://ftpmirror.gnu.org/readline/readline-8.0.tar.gz#e339f51971478d369f8a053a330a190781acb9864cf4c541060f12078948e461" mac_readline --if has_broken_mac_readline
-install_package "stackless-3.7.5-slp" "https://github.com/stackless-dev/stackless/archive/v3.7.5-slp.tar.gz#e2562a8d235adc19be5451c170837f53ef916aec4cd5cd17d9e0ab1f1b875d3f " ldflags_dirs standard verify_py37 ensurepip
+install_package "stackless-3.7.5-slp" "https://github.com/stackless-dev/stackless/archive/v3.7.5-slp.tar.gz#e2562a8d235adc19be5451c170837f53ef916aec4cd5cd17d9e0ab1f1b875d3f" ldflags_dirs standard verify_py37 ensurepip