Make setuptools-scm look for versions only in v[digit] prefixed tags

[matt-graham]

Should resolve #1561

Changes setuptools_scm configuration option git_describe_command from default value which uses glob pattern *[0-9]* to match possible versions from tags to instead use more specific glob pattern v[0-9]* which will only match tags which start with v then a digit which should hopefully avoid spurious matches such as to prof1 which was causing issue in #1561. With this change running python -m setuptools_scm locally succeeds (while was previously failing) as does running tox -e check.