Chandan Singh authored Dec 14, 2018 and Chandan Singh committed Dec 20, 2018

Use raw strings for regex searches, which is the preferred way to do
regular expressions in Python.

Without this patch, currently we get the following warnings:

```
tests/frontend/logging.py:44
  /builds/BuildStream/buildstream/dist/buildstream/tests/frontend/logging.py:44: DeprecationWarning: invalid escape sequence \[
    m = re.search("\[\d\d:\d\d:\d\d\]\[\]\[\] SUCCESS Checking sources", result.stderr)

tests/frontend/logging.py:80
  /builds/BuildStream/buildstream/dist/buildstream/tests/frontend/logging.py:80: DeprecationWarning: invalid escape sequence \d
    m = re.search("\d\d:\d\d:\d\d,\d\d:\d\d:\d\d.\d{6},\d\d:\d\d:\d\d,,,SUCCESS,Checking sources", result.stderr)
```

Chandan Singh authored Dec 14, 2018 and Chandan Singh committed Dec 20, 2018

Specify flags at the start of the expression as per the recommendation
of the standard library.

Without this patch, we currently get the following warning:

```
tests/examples/junctions.py::test_open_cross_junction_workspace
  /builds/BuildStream/buildstream/dist/buildstream/buildstream/utils.py:213: DeprecationWarning: Flags not at the start of the expression '\\/[^/]*\\Z(?ms)'
    regexer = re.compile(expression)
```

Fix Deprecation warnings from regex module

See merge request !1010