refactor: bump pybind version to 2.13.6 #391

Job	Run time
macos-latest, cmake, gcc, easy backend	33s
macos-latest, cmake, gcc, gmp backend	46s
macos-latest, cmake, clang, easy backend	56s
macos-latest, cmake, clang, gmp backend	35s
macos-latest, autotools, gcc, easy backend	1m 5s
macos-latest, autotools, gcc, gmp backend	1m 8s
macos-latest, autotools, clang, easy backend	1m 18s
macos-latest, autotools, clang, gmp backend	1m 17s
ubuntu-20.04, cmake, gcc, easy backend	1m 41s
ubuntu-20.04, cmake, gcc, gmp backend	1m 29s
ubuntu-20.04, cmake, clang, easy backend	1m 52s
ubuntu-20.04, cmake, clang, gmp backend	1m 25s
ubuntu-20.04, autotools, gcc, easy backend	2m 11s
ubuntu-20.04, autotools, gcc, gmp backend	2m 43s
ubuntu-20.04, autotools, clang, easy backend	1m 57s
ubuntu-20.04, autotools, clang, gmp backend	1m 56s
	22m 52s

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