setup: bump mako minimum version to 0.9.1 to get py3 support

Avoid test_api_get_pullrequest crash:

data/env3/lib/python3.7/site-packages/mako/template.py:653: in _compile_text
code = compile(source, cid, 'exec')
E File "email_templates_button_html", line 15
E def render_body(context,url,title='',padding_top=,padding_bottom=,**pageargs):
E ^
E SyntaxError: invalid syntax