[FIX] requirements: improve python3.7 compatibility

Since af9d6b86 and
5a57c29f , python3.7 can be used to run
Odoo. This commit ensures that python3.7 compatibles versions of gevent,
greenlet and lxml will be installed without breaking compatibility with
Debian Stretch packages versions when using the Debian packaged Python.

Closes #25841

requirements.txt

@@ -3,11 +3,14 @@ decorator==4.0.10
 docutils==0.12
 ebaysdk==2.1.5
 feedparser==5.2.1
-gevent==1.1.2 ; sys_platform != 'win32'
-greenlet==0.4.10
+gevent==1.1.2 ; sys_platform != 'win32' and python_version < '3.7'
+gevent==1.3.4 ; sys_platform != 'win32' and python_version >= '3.7'
+greenlet==0.4.10 ; python_version < '3.7'
+greenlet==0.4.13 ; python_version >= '3.7'
 html2text==2016.9.19
 Jinja2==2.8
-lxml==3.7.1 ; sys_platform != 'win32'
+lxml==3.7.1 ; sys_platform != 'win32' and python_version < '3.7'
+lxml==4.2.3 ; sys_platform != 'win32' and python_version >= '3.7'
 lxml ; sys_platform == 'win32'
 Mako==1.0.4
 MarkupSafe==0.23