[python-qemu-qmp MR #13] Sphinx: improve version detection for SDists

[GitLab Bot]

Author: John Snow - https://gitlab.com/jsnow
Merge Request: 
https://gitlab.com/qemu-project/python-qemu-qmp/-/merge_requests/13
... from: jsnow/python-qemu-qmp:doc-version-fix
... into: qemu-project/python-qemu-qmp:main

When building docs from SDist files, we won't have git metadata
available. Use any tools we may have at our disposal to determine the
package version when possible in these cases.

Note: this patch avoids trying to blindly import "qemu.qmp" in just such
a case that the user's environment already has such a package installed:
we want to query specifically the version from the unpackaged source
files.

Fixes #27

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