[cleanup] Fix some flake8 errors

[tox] Bump flake8 version to 3.5 Previous patches fixed new errors raised in this version.

[pyramid] Get rid of a couple of bare "except"

[pyramid] Get rid of a bare "except" in UpdateLoginTimeAuthenticationPolicy Also refactor a bit the method to restrict the try/except block to the code for which we actually want to catch an exception (i.e. the cnx.execute() + cnx.commit()).

[web] Avoid bare "except" in ConcatFilesHandler.concat_cached_filepath()

[test] Use ignore_errors=True in shutil.rmtree instead of try/except/pass

[server] Get rid of a bare except in sqlutils

[server] Do not use logilab.common.date.strptime in sqlutils This function is aliased to datetime.datetime.strptime() and exists only for compatibility with Python <= 2.5 which we no longer support.

[dataimport/test] Ignore flake8 error E741 in test_pgstore.py This error is about "l" being an ambiguous variable name. Here, it's arguably acceptable.

[test] Avoid ambiguous variable name "l" in unittest_req.py This is an error (E741) from flake8 3.5.

[tox] Bound flake8 version to >=3.4, <3.5 The code base is not clean with 3.5, so keep using 3.4 in *stable* branches.

[pyramid] set waitress url_scheme to https when base-url scheme is https See https://docs.pylonsproject.org/projects/waitress/en/latest/#using-behind-a-reverse-proxy Since we do not configure trusted_proxy waitress ignore X-Forwarded-Proto header and may generate http urls (pyramid request.url) when the instance is behind a https reverse proxy. This cause cubicweb-signredrequest to not work with since it rely on the url (including scheme). Set url_scheme to 'https' when CubicWeb base-url scheme is https as a workaround.