Install missing mypy stubs

Mypy currently warns about missing stubs to analyse LNT's code:
lnt/server/ui/globals.py:5: error: Library stubs not installed for
"flask" (or incompatible with Python 2.7)
lnt/server/ui/decorators.py:1: error: Library stubs not installed for
"flask" (or incompatible with Python 2.7)
lnt/lnttool/convert.py:2: error: Library stubs not installed for "click"
(or incompatible with Python 2.7)
lnt/lnttool/common.py:2: error: Library stubs not installed for "click"
(or incompatible with Python 2.7)
lnt/lnttool/import_report.py:1: error: Library stubs not installed for
"click" (or incompatible with Python 2.7)
lnt/lnttool/admin.py:4: error: Library stubs not installed for "click"
(or incompatible with Python 2.7)

This commits adds the necessary dependency to get rid of those warnings.

Reviewed By: PrzemekWirkus

Differential Revision: https://reviews.llvm.org/D104706

Author: Thomas Preud'homme

tox.ini

@@ -19,6 +19,12 @@ skip_install = true
 deps =
     mypy
     sqlalchemy-stubs
+    types-Flask
+    types-Jinja2
+    types-PyYAML
+    types-Werkzeug
+    types-click
+    types-pkg_resources
 commands =
     # No where close to passing yet, but nice to have.
     - mypy --junit-xml=junit-{envname}.xml --py2 --ignore-missing-imports lnt