…security issues.

…11 or 2.9.12.

See https://bugs.launchpad.net/lxml/+bug/1928795

Also add a warning about libxml2 2.9.11/12.

Use the return value altogether to check the subprocess execute
successfully or not as in some case it will print some noise
message though run successfully as below.

 # python
 Python 3.8.10 (default, Nov 26 2021, 20:14:08)
 [GCC 9.3.0] on linux
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import subprocess
 >>> cmd = "pkg-config --modversion libxml-2.0"
 >>> p = subprocess.Popen(cmd, shell=True,stdout=subprocess.PIPE, stderr=subprocess.PIPE)
 >>> stdout_data, errors = p.communicate()
 >>> print(stdout_data)
 b'2.9.12\n'
 >>> print(errors)
 b'do_ypcall: clnt_call: RPC: Unable to send; errno = Network is unreachable\n'

…ve environments.

See https://bugs.launchpad.net/lxml/+bug/1958539

Use "PyOS_FSPath()" if available (Py3.6+). Otherwise, manually check for "__fspath__", in case an object defines it.

W3C specification for xsd:double says
> The special values positive and negative infinity and
> not-a-number have lexical representations INF, -INF and NaN,
> respectively.

Thus case matters. The previously used float.__repr__ would generate
"inf", "-inf", "nan". Now we prepend special handling to get
"INF", "-INF", "NaN" instead (which is still pytype compatible).

Includes minor non-functional alignments of related bool to text code,
and tests to assert its XML schema conformance as well.

Closes https://bugs.launchpad.net/lxml/+bug/1960731