Raise an error if GetCapabilities requests don't return XML

[geographika]

Currently if a GetCapabilities request doesn't return an XML document it raises cryptic error messages such as:

  File "owslib\feature\wfs200.py", line 164, in _buildMetadata
    for elem in self._capabilities.find(nspath("OperationsMetadata"))[:]:
TypeError: 'NoneType' object is not subscriptable

This pull request checks to see if the server response Content-Type is XML, and if not it raises an exception that includes the URL and response. If the response is HTML it attempts to parse the body of the document to use as the error message. This is particularly useful when working with MapServer which returns errors in text/html format. E.g.

http:///example.org/?service=WFS&request=GetCapabilities&version=2.0.0 responded 
with Content-Type 'text/html': 'msCGILoadMap(): Web application error. CGI variable "map" is not set.'

Currently any Content-Type which has any match/partial match against the following list will be considered as XML: ['text/xml', 'application/xml', 'application/vnd.ogc.wms_xml']

It may be better to just check for xml in case there are types I've missed?

In addition the strip_bom was only previously used for WMS GetCapabilities requests - this change means it is applied to all service types (WFS, SOS, WCS, WMTS).

[coveralls]

Coverage: 59.561% (+0.04%) from 59.523% when pulling 7890a58 on geographika:getcapabilities-errors into 306ceed on geopython:master.

[github-actions]

This Pull Request has been inactive for 90 days. In order to manage maintenance burden, it will be automatically closed in 7 days.

[coveralls]

coverage: 60.135% (+0.03%) from 60.101%
when pulling 95be08c on geographika:getcapabilities-errors
into a713c62 on geopython:master.

[geographika]

Rebased against main - all tests passing.

[geographika]

Failing CI is due to not being able to submit to coveralls. Seems unrelated to this change.

 Submitting coverage to coveralls.io...
Error running coveralls: Could not submit coverage: 422 Client Error: Unprocessable Entity for url: https://coveralls.io/api/v1/jobs
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.11.10/x64/lib/python3.11/site-packages/coveralls/api.py", line 298, in submit_report
Received 422 submitting job via Github Actions. By default, coveralls-python uses the "github" service name, which requires you to set the $GITHUB_TOKEN environment variable. If you want to use a COVERALLS_REPO_TOKEN instead, please manually override $COVERALLS_SERVICE_NAME to "github-actions". For more info, see https://coveralls-python.readthedocs.io/en/latest/usage/configuration.html#github-actions-support
    response.raise_for_status()
  File "/opt/hostedtoolcache/Python/3.11.10/x64/lib/python3.11/site-packages/requests/models.py", line 1024, in raise_for_status
    raise HTTPError(http_error_msg, response=self)