Use a recent enough node-gyp release (preferably the system one)

[linkmauve]

Describe the bug
Building fails when no python2 binary is available.

My system ships node-gyp version 6.1.0, which makes me wonder why Converse says it’s building with 3.8.0, while the first version supporting python3 was 5.0.3 according to nodejs/node-gyp#193

To Reproduce
Steps to reproduce the behavior:

1. Run make build

Expected behavior
Converse builds properly, using my system node-gyp.

Screenshots
N/A

Environment (please complete the following information):

• Desktop or Mobile? Server
• Browser [e.g. Chrome, Firefox] N/A
• Converse.js version [e.g. 4.2.0] 8d81637

Additional context
Here is the relevant error in the build log:

gyp ERR! configure error 
gyp ERR! stack Error: Command failed: /usr/bin/python -c import sys; print "%s.%s.%s" % sys.version_info[:3];
gyp ERR! stack   File "<string>", line 1
gyp ERR! stack     import sys; print "%s.%s.%s" % sys.version_info[:3];                                                                                                                                              
gyp ERR! stack                       ^                                                                                                                                                                               
gyp ERR! stack SyntaxError: invalid syntax                                                                                                                                                                           
gyp ERR! stack                                                                                                                                                                                                       
gyp ERR! stack     at ChildProcess.exithandler (child_process.js:303:12)                                                                                                                                             
gyp ERR! stack     at ChildProcess.emit (events.js:315:20)
gyp ERR! stack     at maybeClose (internal/child_process.js:1026:16)
gyp ERR! stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:286:5)
gyp ERR! System Linux 4.19.91-1-lts
gyp ERR! command "/usr/bin/node" "/home/linkmauve/packages/aur/conversejs-git/src/converse.js/node_modules/node-sass/node_modules/node-gyp/bin/node-gyp.js" "rebuild" "--verbose" "--libsass_ext=" "--libsass_cflags=
" "--libsass_ldflags=" "--libsass_library="
gyp ERR! cwd /home/linkmauve/packages/aur/conversejs-git/src/converse.js/node_modules/node-sass
gyp ERR! node -v v13.12.0
gyp ERR! node-gyp -v v3.8.0
gyp ERR! not ok 

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[jcbrand]

Python2 is no longer a dependency for building docs.