[pbs] Add support for series

[steelbrain]

Please follow the guide below

• You will be asked some questions and requested to provide some information, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your issue (like that [x])
• Use Preview tab to see how your issue will actually look like

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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.01.05. If it's not read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2017.01.05

Before submitting an issue make sure you have:

• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

Add -v flag to your command line you run youtube-dl with, copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

$ youtube-dl -v <your command line>
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.01.05
[debug] Python version 2.7.12 - Linux-4.4.0-45-generic-x86_64-with-Ubuntu-16.04-xenial
[debug] exe versions: none
[debug] Proxy map: {}
...

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Description of your issue, suggested solution and other information

root@my-server:/tmp# youtube-dl "http://www.pbs.org/wgbh/masterpiece/programs/series/sherlock-s4/" -F
[pbs] Downloading JSON metadata
[pbs] sherlock-s4: Downloading webpage
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/local/bin/youtube-dl/__main__.py", line 19, in <module>
  File "/usr/local/bin/youtube-dl/youtube_dl/__init__.py", line 444, in main
  File "/usr/local/bin/youtube-dl/youtube_dl/__init__.py", line 434, in _real_main
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 1791, in download
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 694, in extract_info
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/common.py", line 359, in extract
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/pbs.py", line 443, in _real_extract
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/pbs.py", line 425, in _extract_webpage
  File "/usr/lib/python2.7/re.py", line 141, in match
    return _compile(pattern, flags).match(string)
TypeError: expected string or buffer

[ralfharing]

PBS was working a week ago. It appears to be working again now with v 2017.1.10.