Relax proc-macro2 dependency #9

Job	Run time
Miri /	34s
Miri / --release	35s
Doc	15s
Clippy	15s
Features /	14s
Features / derive	14s
Format	11s
Test / x86_64-pc-windows-msvc	52s
Features / std	14s
Toolchain / stable	16s
Test / x86_64-pc-windows-msvc --release	56s
Features / std derive	15s
Toolchain / stable --release	15s
Toolchain / beta	13s
Cross / i686-unknown-linux-gnu	1m 10s
Toolchain / beta --release	14s
Toolchain / nightly	14s
Cross / i586-unknown-linux-gnu	1m 5s
Toolchain / nightly --release	16s
Cross / armv7-unknown-linux-gnueabihf	1m 10s
Cross / aarch64-unknown-linux-gnu	1m 6s
Cross / thumbv6m-none-eabi	1m 5s
	11m 39s

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