Pin Python requirements

Commands used
```
cat requirements.txt | \
perl -ne '
  if (/^(\w+.*) \\/) { $d = $1 }
  if (defined $d && /\s+#.* -r [\w\/]+\.in$/) { $c{$d}=1; undef $d }
  END { print join "\n", sort keys %c }
' >> constraints.txt && make relock_venv
```

CMK-24122

Change-Id: I1f6065db04b86603edad3f661e44d87238548b80

Author: Synss