fix: relative config path error fix

[slashformotion]

fixes #168

It works with relative path

(tbump-py3.12) ➜  tbump git:(main) ✗ tbump --config ./../tbump/tbump.toml 0.10.1 --only-patch
:: Bumping from 6.11.0 to 0.10.1
/home/slash/CODE/github.com/slashformotion/tbump
../tbump/tbump.toml
/home/slash/CODE/github.com/slashformotion/tbump/../tbump/tbump.toml
=> Would update current version in /home/slash/CODE/github.com/slashformotion/tbump/../tbump/tbump.toml
/home/slash/CODE/github.com/slashformotion/tbump
../tbump/tbump.toml
/home/slash/CODE/github.com/slashformotion/tbump/../tbump/tbump.toml
- /home/slash/CODE/github.com/slashformotion/tbump/../tbump/tbump.toml:6 current = "6.11.0"
+ /home/slash/CODE/github.com/slashformotion/tbump/../tbump/tbump.toml:6 current = "0.10.1"
=> Would patch these files
- pyproject.toml:6 version = "6.11.0"
+ pyproject.toml:6 version = "0.10.1"
- tbump/cli.py:21 TBUMP_VERSION = "6.11.0"
+ tbump/cli.py:21 TBUMP_VERSION = "0.10.1"
:: Looking good? (y/N)
> n
Error: Canceled by user

[dmerejkowsky]

Thanks for the fix !