Bitcoin Core requires python 3.5 at least. However, due to travis upsteam changes in the image, the installation of python3 no longer succeeds with brew. See e.g. https://travis-ci.org/bitcoin/bitcoin/jobs/631373413#L252
This should be fixed somehow.
Relevant copy of the log:
0==> Upgrading python3
1
2==> Downloading https://homebrew.bintray.com/bottles/python-3.7.6_1.mojave.bottl
3
4==> Downloading from https://akamai.bintray.com/64/643d627c2b4fc03a3286c397d2992
5
6######################################################################## 100.0%
7
8==> Pouring python-3.7.6_1.mojave.bottle.tar.gz
9
10Error: The `brew link` step did not complete successfully
11
12The formula built, but is not symlinked into /usr/local
13
14Could not symlink Frameworks/Python.framework/Headers
15
16Target /usr/local/Frameworks/Python.framework/Headers
17
18is a symlink belonging to python@2. You can unlink it:
19
20 brew unlink python@2
21
22To force the link and overwrite all conflicting files:
23
24 brew link --overwrite python
25
26To list all files that would be deleted:
27
28 brew link --overwrite --dry-run python
29
30Possible conflicting files are:
31
32/usr/local/Frameworks/Python.framework/Headers -> /usr/local/Cellar/python@2/2.7.17/Frameworks/Python.framework/Headers
33
34/usr/local/Frameworks/Python.framework/Python -> /usr/local/Cellar/python@2/2.7.17/Frameworks/Python.framework/Python
35
36/usr/local/Frameworks/Python.framework/Resources -> /usr/local/Cellar/python@2/2.7.17/Frameworks/Python.framework/Resources
37
38/usr/local/Frameworks/Python.framework/Versions/Current -> /usr/local/Cellar/python@2/2.7.17/Frameworks/Python.framework/Versions/Current
39
40==> /usr/local/Cellar/python/3.7.6_1/bin/python3 -s setup.py --no-user-cfg insta
41
42==> /usr/local/Cellar/python/3.7.6_1/bin/python3 -s setup.py --no-user-cfg insta
43
44==> /usr/local/Cellar/python/3.7.6_1/bin/python3 -s setup.py --no-user-cfg insta
45
46==> Caveats
47
48Python has been installed as
49
50 /usr/local/bin/python3
51
52Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to
53
54`python3`, `python3-config`, `pip3` etc., respectively, have been installed into
55
56 /usr/local/opt/python/libexec/bin
57
58If you need Homebrew's Python 2.7 run
59
60 brew install python@2
61
62You can install Python packages with
63
64 pip3 install <package>
65
66They will install into the site-package directory
67
68 /usr/local/lib/python3.7/site-packages
69
70See: https://docs.brew.sh/Homebrew-and-Python
71
72==> Summary
73
74🍺 /usr/local/Cellar/python/3.7.6_1: 3,977 files, 60.8MB
75
76Removing: /usr/local/Cellar/python/3.7.5... (4,049 files, 61.9MB)
77
78Removing: /Users/travis/Library/Caches/Homebrew/python--3.7.5.mojave.bottle.tar.gz... (14.8MB)
