lint: Fix problematic merging of pyflakes stdout/stderr.
lint: Fix problematic merging of pyflakes stdout/stderr.

We were getting pyflakes lint error output without line numbers like
this:

pyflakes  |     if user_profile.is_realm_admin and
pyflakes  |                                       ^
pyflakes  |

Apparently the cause was that stdout and stderr was getting mixed
badly, creating "unused import"s lines that had the first of that
error (containing the line number) just above.

As a result, printing out the lines of output from pyflakes' merged
stdout/stderr feed looked like this:

b"zproject/settings.py:95: 'from .prod_settings import *' used; unable to detect undefined nameszerver/views/users.py:49:39: invalid syntax\n"

Note the lack of newline in between the end of the first error at
"names" and the start of the second at "zerver".

This appears to be a change in Pyflakes behavior when we switched to
Python 3; probably they're missing a flush() somewhere.

dhcrzf/zulip,timabbott/zulip,dhcrzf/zulip,brockwhittaker/zulip,mahim97/zulip,jackrzhang/zulip,brainwane/zulip,brainwane/zulip,punchagan/zulip,shubhamdhama/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,Galexrt/zulip,synicalsyntax/zulip,amanharitsh123/zulip,hackerkid/zulip,hackerkid/zulip,verma-varsha/zulip,andersk/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,punchagan/zulip,rht/zulip,mahim97/zulip,rht/zulip,dhcrzf/zulip,hackerkid/zulip,verma-varsha/zulip,kou/zulip,shubhamdhama/zulip,verma-varsha/zulip,eeshangarg/zulip,rishig/zulip,andersk/zulip,rishig/zulip,jackrzhang/zulip,tommyip/zulip,dhcrzf/zulip,showell/zulip,mahim97/zulip,brockwhittaker/zulip,synicalsyntax/zulip,brockwhittaker/zulip,eeshangarg/zulip,verma-varsha/zulip,mahim97/zulip,zulip/zulip,shubhamdhama/zulip,zulip/zulip,jackrzhang/zulip,dhcrzf/zulip,kou/zulip,mahim97/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,brockwhittaker/zulip,shubhamdhama/zulip,brainwane/zulip,rishig/zulip,rishig/zulip,timabbott/zulip,synicalsyntax/zulip,kou/zulip,hackerkid/zulip,rht/zulip,synicalsyntax/zulip,brockwhittaker/zulip,rishig/zulip,brainwane/zulip,dhcrzf/zulip,zulip/zulip,shubhamdhama/zulip,rht/zulip,Galexrt/zulip,amanharitsh123/zulip,punchagan/zulip,kou/zulip,andersk/zulip,amanharitsh123/zulip,Galexrt/zulip,jackrzhang/zulip,rht/zulip,zulip/zulip,andersk/zulip,rishig/zulip,synicalsyntax/zulip,punchagan/zulip,andersk/zulip,tommyip/zulip,zulip/zulip,Galexrt/zulip,rht/zulip,punchagan/zulip,rishig/zulip,timabbott/zulip,kou/zulip,tommyip/zulip,synicalsyntax/zulip,hackerkid/zulip,shubhamdhama/zulip,amanharitsh123/zulip,eeshangarg/zulip,showell/zulip,jackrzhang/zulip,mahim97/zulip,zulip/zulip,hackerkid/zulip,tommyip/zulip,Galexrt/zulip,showell/zulip,showell/zulip,Galexrt/zulip,eeshangarg/zulip,eeshangarg/zulip,jackrzhang/zulip,eeshangarg/zulip,verma-varsha/zulip,jackrzhang/zulip,timabbott/zulip,andersk/zulip,timabbott/zulip,kou/zulip,brockwhittaker/zulip,kou/zulip,verma-varsha/zulip,brainwane/zulip,brainwane/zulip,tommyip/zulip,Galexrt/zulip,amanharitsh123/zulip,timabbott/zulip,showell/zulip,amanharitsh123/zulip,tommyip/zulip,tommyip/zulip,dhcrzf/zulip,brainwane/zulip,shubhamdhama/zulip