remove dependency on strum (as unused) and bump strum_macros to 0.25

[klensy]

to switch dependency of syn from 1.* to 2.* version.

strum-macros new msrv is 1.56 (Peternator7/strum#261) but i can't find spdx-rs msrv, so this up to reviewer.

[klensy]

Funny thing: on my win machine there two failing tests, even on master branch:

---- parsers::test_super::file_info_is_retrieved stdout ----
thread 'parsers::test_super::file_info_is_retrieved' panicked at src\parsers\mod.rs:920:9:
assertion `left == right` failed
  left: Some("The concluded license was taken from the package level that the file was included in.\r\nThis information was found in the COPYING.txt file in the xyz directory.")
 right: Some("The concluded license was taken from the package level that the file was included in.\nThis information was found in the COPYING.txt file in the xyz directory.")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- parsers::test_super::spdx_creation_info_is_retrieved stdout ----
thread 'parsers::test_super::spdx_creation_info_is_retrieved' panicked at src\parsers\mod.rs:782:9:
assertion `left == right` failed
  left: Some("This package has been shipped in source and binary form.\r\nThe binaries were created with gcc 4.5.1 and expect to link to\r\ncompatible system run time libraries.")
 right: Some("This package has been shipped in source and binary form.\nThe binaries were created with gcc 4.5.1 and expect to link to\ncompatible system run time libraries.")


failures:
    parsers::test_super::file_info_is_retrieved
    parsers::test_super::spdx_creation_info_is_retrieved

[mmurto]

Thanks @klensy ! No real decision has been made on MSRV. I created an issue at #30, feel free to comment if you have thoughts. As 1.56 is decently old, I will release this without considering this a breaking change. Should decide on MSRV related matters before 1.0.0.