Switch to following semver spec for tag names?

According to [semver spec](https://semver.org/):

> 9. A pre-release version MAY be denoted by appending a hyphen and a series of dot separated identifiers immediately following the patch version. Identifiers MUST comprise only ASCII alphanumerics and hyphens [0-9A-Za-z-]. Identifiers MUST NOT be empty. Numeric identifiers MUST NOT include leading zeroes. Pre-release versions have a lower precedence than the associated normal version. A pre-release version indicates that the version is unstable and might not satisfy the intended compatibility requirements as denoted by its associated normal version. Examples: 1.0.0-alpha, 1.0.0-alpha.1, 1.0.0-0.3.7, 1.0.0-x.7.z.92, 1.0.0-x-y-z.--.
>
> 10. Build metadata MAY be denoted by appending a plus sign and a series of dot separated identifiers immediately following the patch or pre-release version. Identifiers MUST comprise only ASCII alphanumerics and hyphens [0-9A-Za-z-]. Identifiers MUST NOT be empty. Build metadata MUST be ignored when determining version precedence. Thus two versions that differ only in the build metadata, have the same precedence. Examples: 1.0.0-alpha+001, 1.0.0+20130313144700, 1.0.0-beta+exp.sha.5114f85, 1.0.0+21AF26D3----117B344092BD.

According to [PEP 440](https://peps.python.org/pep-0440/):

> A release version segment is either a `.` separated sequence of integers or a sequence of integers separated by zero or more additional `-` or `_` characters. Public versions MUST comply with the following scheme: `[N!]N(.N)*[{a|b|rc}N][.postN][.devN]` where all the elements in square brackets are optional. Pre-release versions bear a `a`, `b`, or `rc` prefix followed by an integer. Examples: 1.0.0a1, 1.2.3rc1, 1.2.3, 1.2.3.post1, 1.2.3.post2.dev3.

We have been compliant with PEP 440.

However, we are non-compliant with semver by tagging releases as `v0.12.1b11` (should be `v0.12.1-beta.11`). This has led to minor extra work in CI since the version-checker action we used started requring following the spec strictly, and the old version has lots of deprecation warnings. [We forked the action](https://github.com/ActivityWatch/check-version-format-action), but we'd rather return to using the upstream version.

The PEP 440 and semver specs are incompatible due to how alpha/beta/rcs need to be written: PEP440 disallows dashes, semver requires them. Apart from that, they both allow for extended metadata `+` suffixes ([Local version identifiers](https://peps.python.org/pep-0440/#local-version-identifiers) in PEP440 terms).

Not sure what's the right call here. It's a little bit of work changing build scripts etc, but work is coming anyway with the extra maintenance from non-complicance as semver has better support in non-Python tooling.

If we decide it's a good idea, we could do the switch for `v0.13.0`.

Not in a rush to make a decision, but I've been thinking about it for a while, and needed to write it down.